Ok, so I know you dont like me, but hear me out. 15 is still a kid. I know, she seems more mature, but she's just a kid. I look back to when I was a 15 year old girl, and back then, (even though ive never had a bf), I thought I was ready for what the world had to offer. i thought older guys would be better. Ive changed so much from 15 to 18 that I dont even recognize the person that I was at 15 as myself. And Im still young.
Ive been seeing a dude (not dating, but ya know), and he's 6 years older than me, or rather, 5 years and 5 months older than me. And he is a great friend, a great guy... I never felt this way about anyone before. Not even all the other guys Ive ever talked to or formed some sort of attachment to. He's been all my firsts, except wont sleep with me.
I can see where you are coming from, how much you want her well, but unless she's my age or older, she wont be able to handle the reality of being with someone much older. There is a certain kind of weird disconnect when my 24 year old man friend talks about going out to bars, etc... where as Im approaching 19 and havent lived that much yet.
Please give it and her time, she will change.
but you're not a bad man for wanting her