Im 18 and just got out of a long term relationship with a girl who was 23. We lived togeether, loved eachother, but I recently packed up my stuff to move to Hollywood (I am not a star eyed actor, btw). Anyways it was fine, as long as you get along and can relate to eachother, age shouldnt matter. Granted, it was interesting being with a 21 yo girl when i was 16, but i guess i was just lucky
edit: also i guess the main advice is to try and stay away from 21+ venues and events (obviously). I am kinda hyper sensitive maybe, but it makes me (maybe him too) feel like a little kid when you cant get in somewhere. This should be pretty easy unless you absolutley have to drink alcohol in certain public places.