In one word, Yes.
I used to have a friend who had a similar relationship. It ended up alienating her from everyone. She quit school, moved in with him. Last I heard they were trying for a baby. She's barely 17! Just think of that aspect, he's going to want kids soon. Are you really wanting to give up your teenaged years to have a kid? But hey, if you want the same age gap between you and your kids as your boyfriend, go ahead. When people see you around they'll assume baby, mother, grandfather... and if that doesn't put you off: ew.