17 is different than 15 in my opinion. Especially since they're still together. That's a relationship, not just hooking up or taking advantage of someone younger because they think you look cool when you drink.
NO! You're just asking for her parents to report you after they found out you had sex with her and then you're stuck with the label of being a child molester for the rest of your life. It happened to one of my good guy friends. If she was of the 17 year old or 18 year old age the hell yeah go for it. But if she is 15 you're just asking for trouble. And plus do you really want to be with a 15 year old girl, I am sure she can't be that mature. You're 20 years old, you should be able to get girls your own age...
16, 5 years difference for 14 and 15, 2 years difference for 12 and 14. anal legal when both parties older than 18 or if they're married (except when more than 2 people are involved, in which case it's illegal)
Like 18 so you can't get labeled with statutory rape if her parents find out. But in the meantime get to know her, and get closer to her so when the time comes you can snatch that up no problem since she already knows you