I was 21 when I started my master's degree, so I can relate to being the youngest person in a batch. I don't know how much use my advice may be, as a master's, rather than Ph.D. student, but I'll try anyway. First of all, congratulations on your acceptances. In my opinion, your age won't make a difference. It obviously didn't to the admissions commitees. Also, based on my experience, I don't think your social interactions with your peers would be altered at all due to your age. I imagine most people start their Ph.D. from 22-25 years of age, and it's not like you're 16 or something.
I see there have already been very thorough posts made in reply to your questions, so a lot of what I am about to say may be repetitive, but here goes.
Selecting a research topic: One suggestion is to try and choose a research topic which has reasonably wide appeal in the field that you intend to pursue your career in. You don't have to pursue a career in whatever you do your dissertation research on, but obviously it would be great when you apply for jobs if your dissertation fits with what your potential employers do. You have time to think about what you want to do after your Ph.D., whether you want to pursue academia or industry, etc., but try to give it a bit of thought.You don't want to pigeonhole yourself by working on an obscure topic which would be appreciated by only a few specialists in the field, and thus limit your job prospects. Picking a research project which would be of significant interest or importance to the field will also help in the short-term, as you'll have a wider pool of professors to choose from to comprise your dissertation committee. Don't be like me, doing a basic molecular biology thesis project despite being an engineering student, and having to scramble to find committee members from my department who have at least passing interest in what I do. Another very important thing is to select a doable project. It's easy to pick the most challenging project, thinking you have so much time to work on it, only to get inconclusive results and find yourself scrambling at the end. Even the best planned project may look great on paper, but when you actually go around to doing it, you can get all sorts of setbacks you had never foreseen. For example, stuff which had been working before can inexplicably stop working, such as genomic DNA purification kits, molecular cloning, and sequencing reactions (drawing from my own experience). You can find yourself spending a lot of time on troubleshooting simple problems for even the best thought-out projects, so don't stress yourself out by being too ambitious in your project choice. Obviously you don't want to do pedestrian, barely original research, but strike a balance. A possible approach would be to pick a high-risk, high-reward topic, but have a less glamorous, "safety net" project as backup. This is what developmental biologist Leonard Zon of Harvard advises his graduate students and post-doctoral researchers. I'm not saying don't pick an interesting project or be afraid of challenges, but be realistic as to what you can accomplish in the timeframe that you have, and have a Plan B if possible. And of course, the main thing is to choose a research topic which you have a passion for; you will likely be spending the next 4-6 years of your life working on this. If you will be doing lab work, laboratory rotations will be extremely important in this process. Three to four weeks of working in a lab will give you a sense of what life would be like working in a particular field or subfield.
Selecting an advisor: It goes without saying, but pick an advisor whom you can get along well with. Make sure it is a person you feel like you can communicate candidly with, and who will be candid with you. Again, you'll find out about the personalities and expectations of potential advisors during laboratory rotations. Some advisors expect you to be in the lab at certain times, some advisors could care less when you show up, as long as you get the work done. Some advisors are very hand-on, will provide a lot of advice and suggestions, others are more laissez-faire, and will give you a lot of autonomy. Think about whether you want a lot of flexibility in doing your project, or whether you want to be in a more structured environment. In general, established professors, who often have large labs, tend to let students sink or swim on their own; newer professors, who often have smaller labs, are probably more invested in your success or failure. On the other hand, well-established professors tend to have more resources and funding, and their recommendation will carry a bit more weight when you apply for post-doctoral positions. Also, one thing I would like to say is, don't be afraid to let your opinion be heard. Don't just agree with everything your advisor says. He or she may be the most eminent person in the field, but if you have a disagreement over how an experiment should be done, for example, make sure you voice your concerns.
Selecting a dissertation committee: I've only had experience in selecting a master's thesis committee, but I imagine it would translate to a selecting dissertation committee as well. It's important to select committee members who work well together. As in any workplace, there are people who get along well, and those that don't, so discuss your intended committee composition with your advisor before reaching out to potential committee members. I've personally not had to deal with any personality clashes with my committee members, but I would still say it's something to keep in mind. Also, try to include professors who are prominent in your field of interest. Don't think that a professor is too famous or too important to serve on your dissertation committee. I've heard that one physics Ph.D. student was hesitant to ask the eminent Richard Feynman to serve on his committee, but when he did ask, Feynman readily agreed. Apparently this was the first time someone had ever asked Feynman, because all the students were afraid to thus far. Imagine getting a job recommendation from the Feynman of your field! Having said that, make sure that your committee members are there for a valid reason; select committee members primarily based on the skills and expertise that they bring to the table. If you are doing an epigenetics study, it's far better that you pick the lesser-known expert in chromatin remodeling rather than the world-renowned leader in gold nanoparticles, to use an extreme example.
Interacting with faculty: Obviously, it is important that you should try and build strong relationships with the professors in your department. I'm sure you must have been good at that as an undergraduate, since you would have gotten strong recommendations for graduate school, so what I say may be superfluous to your requirements. I think it's more important, but at the same time easier, to interact with your professors in graduate school. In undergraduate, your main avenue for interaction is through office hours, and your grade in their class is mainly what shapes the professor's impression of you. In graduate school, you'll get to go through laboratory rotations with different professors, and the classes will be much more of the seminar variety, where you interact directly, discussing primary literature with the instructor and your classmates. In one of my graduate seminar courses,(headed by the DGS for the program), I, along with some other people, actively contributed to the class discussion throughout the semester. Others were mostly silent throughout, basically just showing up just for attendance. It didn't affect their grade, but the DGS expressed his disappointment that some people did not seem to show interest in the field that they had ostensibly chosen to pursue for their career. That's obviously not the best way to kick things off in a program you are planning to spend the next few years in. If you have strong relationships with your professors, when the time comes to pick your dissertation committee, get job recommendations, etc. it will be much easier.
Graduate work and studies: DO NOT feel that you have to give your dog away. I know plenty of Ph.D. students with dogs, and they do manage to find the time to spend with their pets. However, based on my experience, you'll have to be a bit flexible at times when it comes to your research, being prepared to work on nights or weekends if necessary. This is especially true if you'll be working in a lab. There will be periods where you need to get a lot of stuff done in a short period of time, but there will also be relative lulls, so be prepared to adjust your schedule accordingly. But you don't have to be in the lab 24/7. The most important factor is your time management. If you are organized and plan ahead, there's no reason why you can't do your research mostly in a 9-5 timespan. Some people do that, others, like me, are haphazard, and come at random times in the night to get work done. So definitely, as long as you manage time well, your life won't be swallowed up by research, and you can devote the time that you need to your dog. As far as coursework, I don't think you need to worry about it occupying an inordinate amount of time. The courses will be more advanced, but since you have been accepted into multiple Ph.D. programs, you are obviously smart enough and talented enough to handle it. The courses will be much more of the seminar-type, involving discussions of primary literature, and your exams will be testing your critical thinking more than requiring you to cough up book knowledge. You will have to do a ton of reading of journal articles for both your research and many of your courses, which of course can be done at home. It might take a bit of getting used to at first, especially since many articles are not exactly lucidly written. Unless you do absolutely atrociously in a course, you'll get A's and B's in your courses, so don't stress about grades too much. You're obviously intelligent, so as long as you put in an honest effort, you'll get your just reward; you don't have to put in superhuman time and effort to get good grades in your coursework.
Non-academic life: The following advice is not stuff which I follow myself (wish I did), but I think it is valid nonetheless. Do not let your graduate work consume your life. Yes, you will have to spend a lot of time and effort on your research and courses, but set a limit. Do not let it prevent you from having a social life, spending time with your dog, etc. If you are someone who wants to have the weekend off, manage your time wisely, as I said before. But if you do find yourself having to spend inordinate amounts of time, just stop, take a step back, and make sure you get your time off. Ph.D. is a marathon, not a sprint, so it's much more important to stay mentally fresh, both for your own sake, and the quality of work that you do. I don't have much of a social life, but that's not because I can't find the time; I'm just simply an insular person. I know Ph.D. students, and they do manage to have time to do stuff, like play intramural sports, spend time with friends, etc. Unlike in undergraduate, where you are focused on coursework, and your schedule is much more rigid, in graduate school, your time is much more flexible; you can make decisions on how to use your time, so you can structure it such that you can have a social life. Just be careful not to procrastinate, because that can come back to bite you. When it comes to friends, I suspect you will make good friends in your cohort, because it is a small group, and they will be going through many of the same experiences that you will be. You can also meet graduate students in other departments, often through mixers and events hosted by your graduate student organization. If you are into sports, you can also make new friends by meeting other people if you play your basketball, tennis, etc. at your school recreation center. And again, your age will definitely not be a problem when making friends, having a social life, etc.
This is about all I have for now, and if I think of anything else I feel is important, I'll post again. Best of luck with graduate school this fall!
Talk to your grad coordinator, supervisor and even a career counsellor at your school. If it were me, I would want a range of opinions and be a bit worried that the departmental people may sway me to one side or another for their own benefit (e.g., it might look better on a department if they have more thesis students...). By talking to a career counsellor, they can suggest the best steps for you to take to get to where you want (i.e., education, job), and it can be specific for your field.
In my experience, thesis base is the way to go for a PhD. There are several reasons (1) experience, as someone mentioned, PhD programs are strongly research based and schools like to see you have experience conducting/producing research independently, and (2) product!!! Not that you can't publish research papers, but you can get 1-2 publications out of your MA thesis, and pubbles looks really great on PhD apps and scholarships!
Good luck!
PS: Personally I hated courses with exams, term papers, readings..blah! When I was done classes I was soo happy! And additional 7 courses would make me cry!
I agree that you should talk to someone in the field about it. I had this same decision to make, and I was told that the major research paper is a better option for me because of the focus in my field relating to your chances at the PhD level on how successful you have been in getting your research published. The logic was that, since I would be taking more courses, I would have more opportunities to develop my ideas and get the papers for the classes published since I would be writing more of them for my degree requirements instead of doing extra work on the side (and we all know there won't be much free time!) to increase my publishing. My supervisor for my MA program was also very happy that I picked the MRP and not the thesis. I can see how this would vary from field to field though, so I would ask someone who's very involved in the graduate program you're entering, or even a trusted professor at the school you're at now. Good luck!
Shari A Williams reacted to Dal PhDer in "learn from each other" - prof naive about competition