I agree with a lot of what has been said here but it looks like the bell curve probably peaks somewhere in the late 20's from the guy's perspective. That's pretty-much what I anticipated.
Agree with Ammonite about the importance of attitude over age. That said, I find that my best experiences have tended to come with ladies over 35--with some spectacular experiences involving the younger crowd.
18-50 for me. No 2 women are the same. I love all listed age groups and have had the pleasure of experiencing each one. Only thing that does bother me is not being able to see certain providers because of their age preference. It makes me want to get old quicker.