I wouldn't get so involved with someone at 15 that it limits you - my friends has been with her boyfriend since she was 13 and they see each other every few days and are quite serious, but they've made loads of decisions because of each other, like her not applying to American unis because he doesn't want to live there. I personally wouldn't want to be pulled back at 16 because of someone I might not stay with for the next year, but they seem pretty serious, so like someone said, it's a very individual thing, I guess.