Hello!
I was 20 when I began my M.Ed. Program and I'm 22 now (I will enter the PhD program I've been accepted to in the fall 2015 term). I am definitely the youngest person in my current cohort and I anticipate that I will be again in the fall. First I would say to you that if you are accepted it is for a reason, and thus your age is just a portion of who you are and what you bring to the table. If they accept you on the graduate level, they are confident that you can contribute to the program in some way, so hold dear to the fact that you belong just as much as the older students! Your perspective will be different, but it is still valuable.
When I first began my program, I tried to hide my age. I felt very conscious about it, and rather awkwardly changed the subject to avoid talking about it whenever the topic arose in conversation. Eventually, one of my birthdays fell on a class day and I was ousted. To my surprise everyone was shocked at how young I was, remarking that they never would've known I'm light of my maturity level and how I contributed to class discussions. I felt very supported by most of my cohort.
However, wasn't always this way. A few members of my cohort made jokes about not being able to go to dinner together because I couldn't drink, or made snide remarks about my lack of experience. After a while the jokes died down and people forgot about the age difference. Now, most people find it admirable that I am so young and have progressed so far in my education.
Bottom line, there are moments when its awkward to be a young grad student. Some people will be jerks about it, but most won't care. On this level, it really shouldn't matter. After all, each of you are sitting in the same classrooms, right?