This question just made me think of something; when I was ten or so years old, I had a friend who was the same age, and he had really hairy arms. I'm sure it was his nationality, he was middle eastern. Just thought I'd share. And I think this phenomenon is quite common, though change in color and intensity of change will vary from person to person.
I think just thicker, maybe some men are not at their peak in their early 20s (Sexually speaking). I mean, I find that a man seems most masculine in his late 20s-40s even. A lot of guys in their early 20s (Though not all) still seem a bit pubescent, testosterone levels probably peak more mid-late 20s and upwards.
That's often why girls are drawn to older men. Me-ow.