When I was 14 my boyfriend was 17 and I can still confidently say it was the best time of my life. Our maturity level was equal, we had a lot in common, and everything was great. Mind you he was 16 for many of the months but still.
It's actually becoming fact that girls mature 3 years faster than males, therefore in mental conditions, they would be the same age. He doesn't want a 14 year old because he wants a 14 year old, he wants her because he's in love with who she is.
I say go for it
i've always been a sucker for older guys myself anyhow, just make sure her parents know.
Good luck brother :)