I am a year older than my gf, and graduating a year sooner. She will be abroad in Asia next semester then when she graduates, she will be in the military so my going abroad for a 3 year PhD program won't really be that much of an issue, as there would not be much visiting happening any way.
We have also been going out for 2.5 years and visit each other about once a month if lucky, and usually do not see one another during the summer.
We have both talked about what will happen and neither of us expect to see the other for the first year, since it will be her sr year and my first year at the PhD program (read: no sleep, no friends). The next two years would depend on where she was stationed, and if we decided to get a courthouse wedding for the spousal benefits and location.