Girls seem to look best from age 17-19, after that, girls seem to fall off and either gain weight, get wrinkly skin, or both by the time they are in their early 20s.
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Honestly, I think it's more of a matter of a person's tastes changing as they age.
I, for example, used to find girl in that age range (17-19) to be the most attractive, just as you've mentioned, and for the same reasons. Anyone older than, say, 25 was just... old.
As I've gotten older, I've come to find women older than that to be more appealing. The older I get, the less attractive younger girls become. Now that I'm 30, I just can't see 19 year olds as attractive, the same way I used to. Now they just look like kids to me. I could never be attracted to a "kid".
My girlfriend is in her 30's, and I think she's the sexiest woman I've ever met, even though, ten years ago, I would have considered someone her age to be "old".2