Is 21 too young for a guy to have a long term relationship?

I've been with my boyfriend for 4 1/2 months and he still acts a bit younger than he is, likes to be social with friends, play video games, and things but only seems to see me once a week sometimes twice. He is 21 and i am 19. He does work a lot and has classes but he doesn't seem to need to see me very much but its always been that way but he treats me great. He was put under a curfew with his dad with video games and texting not permitted after 10 so he will focus more on college work, he flunked his first year on his own and now goes to a community college. He's never had a long term relationship but he pursued me and asked me to be his girlfriend so I just wonder what his motives are. He's never been in love and he didn't get sex from me before being my boyfriend so I just want to know, why do guys who are a inexperienced and still growing up seek out girlfriends?