My current boyfriend is three years younger than I am: I'm 27 and he's 24. But if we had met back when I was 17 or 18, ew! Creepy. What does a 17-year-old girl doing with a 15-year-old boy?
As you age, the maturity differences aren't as vast (although there are plenty of older people of both sexes who still act like they're 12). You go through so many changes between 15 and 17. Mentally, physically, emotionally. When she turns 18, your parents can have her arrested! I'm sure she's a great girl, but you both need to find someone your own age, if you should be dating now at all--that's your parents' call.