There are 20 year old men and women who "look much older". Who are fatter, have bad skin.
There are 40 year old men and women who "look much younger". Are thin, have good skin.
Let us define age as not when a baby is born from the womb-if we follow Dawkins' Selfish gene idea, let's define a human (or an animal or plant) by it's genetic age.
A 20 year old woman looks much older than a 40 year old in our conventional definition of age at present. I argue that the genetic age (a new definition of age) of the 40 year old woman is LESS than that of the 20 year old woman's. She is a "newer gene". That's why that fresh, youthful look. A healthier look.
As I said in a previous post, our weight is an increasing function of our conventional age. But it is even more so for our genetic age.
Same goes for skin quality-wrinkles, for example.
Bodies are just forms taken by genes to propagate themselves (Dawkins), and our conventional definition of age is very imperfect. We can understand humans better by ignoring their date of birth, but focusing on how they really look, and of course how they behave (some guys are mature at 15, others stay immature until 60...)
But purely on physical looks or appearance, we can still see huge deviations between people's conventional age and genetic age.
I believe the a younger genetic age is "healthier". Beauty itself in humans (or animals) is a measure of health-you want to cross with beautiful people because they are in reality healthier, or younger genetic age wise. The beauty is the effect of the good genes, and has low correlation with conventional age. You can think of younger genetic age people as the healthiest genes in the gene pool.
The correlation between conventional age and genetic age is worse for men than women.
See also: Genetic Affinity in Friendships