I'm 27 and if I were 65 or so I wouldn't go for someone younger than 30 at a minimum. People are still a work in progress in their 20's, with all the immaturity and lack of self-knowledge it entails, and moreover a fair number of guys can look great into their 40's or even 50's if they take good care of themselves.
These broader issues have been on my mind, since I feel like in these last couple of years I have gone into another period of my life -- and that has to do a lot with realizing I was getting older. When I was younger I really didn't have too much insight into the workings of my personality and of the world at large, although I was always "book smart".
This process of maturing has made me happier. With getting older and seeing people you love dying, I became aware of my mortality in a way I never could earlier and that made my whole outlook clearer: that each day can be your last, so enjoy each moment, even just mundane things like having tea in the garden, or cooking a meal, or sitting on the metro and looking at people. That also means that I don't care so much about pleasing other people or projecting any image or winning ego contests, or focusing too much on acquiring things. It also makes the little annoyances and dramas of life seem unimportant and often funny. I also focus on things that are really important like my partner or family.
Socializing with some guys 19-21 recently I was reminded of my own insecurities, lack of knowledge, and dramas in that period. I wouldn't want to go back to that, and I wouldn't want to date someone in that phase.