Are you talking about girl/boy, boy/girl, girl/girl or boy/boy? A 15 year old is still a minor so it may not be a good idea for a 19 year old to necessarily be friends as if something bad happens, the 19 year old may be held legally liable. I'm not talking about just sexually, but "contributing to delinquency of a minor" so if the 15 year old is caught, say, shop lifting and blames it on the 19 year old, etc. and the 19 year old is "there" at the same time, it could be unpleasant. I would look for people older than I was (more experienced) or the same age.