Very uneven territory you're walking on here since she is 15. I did the same though, I dated a girl who was 15 and I was 18. I didn't go near her once in a sexual way till after she was legal and both myself and her felt comfortable with it. We started seeing each other about 3 or 4 months away from her 16th birthday. Nice girl, didn't last long though I soon remembered how much I love my Ex/Current girlfriend.