I've never thought age was really an issue, I'm not talking about fifteen year old girls and thirty year old men. I'm talking about the adult world, you know the one I was pushed into. It comes with taxes.
I do wholly agree with this. However, and I mean no offense by this, at 21 I would still consider myself a child. Hell, I passed 21 a while ago, and still do. Your brain doesn't finish until near 27. Of course, people are all different, and some are quicker or slower, but that is the average. Now, if an older person wants to put up with you, good for them, but some people, for whatever reason, can't.
Now you may be a very mature 21 year old, but that isn't normal. I really can't image a mature 21 year old guy. Every single one I have met are just farked. But I have had a biased experience, living in a hilljack heroin town. Maybe she has experiences as I do.
Then just keep it up. As you get older, age matter's less. In ten years time, you will be perfect.
Also, bring up the fact that women normally live longer, so she needs to be with a younger man, and you are the perfect age for that.