I dated a girl who was 18 and in college and I was 39 at the time. She was more mature than most girls in their early 20's.
The age thing is just a guideline of how mature someone will be at that age. Furthermore, maturity is only part of the chemistry. Things in common are much more important than the fact that the girl is close to your age.
I just find it ridiculous that our American society thinks it's weird/wrong when they see a couple that is years apart when most people have adjusted to the fact that we have men fvcking men and women fvcking women and somehow that's okay now but if you're 40 and you're dating a 19 year old, oh well...that's just creepy.
Just goes to show you how fvcked up this country really is.