But there are bits where he kind of suggests he is struggling with his feelings for her which are totally inappropriate. Sounds like she is early teens at the very least.
I may be way off here, but I understand it as an underage girl who tries to look older than she is (would hardly be the first) - not necessarily with any 'inappropriate' intent.
The singer is attracted by what he believes is an adult woman, but he then realises that she isn't an adult, therefore he must step well away; but he is still struggling with the false image of what his mind has now perceived to be an attractive woman, he isn't actually fancying an underage girl.
She has a crush on him, and is too immature to truly comprehend how wrong it is; but he is the adult and thus knows he has to take responsible action, however much his mind deceives him.
He calls her 'young girl' (in fear, on realising, maybe), but she very obviously isn't a little child; she's maybe only two or three years away from when a relationship would be fine, but she isn't yet. If that makes any kind of sense.