In all srsness, 18 to 20-something. There's a reason the supermodel/Playmate/pornstar age-window is what it is.
Pornstars have a bigger window shelf life, because there's a market for milfs and gilfs. Don't let the media and pop culture dictate what beauty is personally to you. One man's skank is another man's angel.
For me 17-21 is what I call the goddess age for girls. That is when their are skinny, young, tight, and if you have game, slutty.
28 - 40 It takes a woman some time to stop following that look, finding themselves and accepting who they are. When they do....its hot. Nothing like a confident beautiful woman.
The older you get the better the young girls look. If I could go back with what I know now I would be banging all of them.