I think part of the looks discussion stems from TV teen shows, I'm afraid, where 25-year-olds act the 15-year-olds. When one turns 20, their body doesn't automagically turn muscled, you know. If one is skinny, he remains skinny; if one is fat, they remain fat. And if one works out (and has the natural predisposition for it), they'll be muscled even at age 15. In order words, body status means little or nothing (in RL, and therefore in SL).
As for height, that is because of the need for a camera view. I had this friend who built a replica of his RL flat in RL dimensions. He invited me over and it was impossibly to move around, unless I was in mouselook mood (which simulates RL eyesight). I think that's what got dimensions distorted in SL and then expanded in body size too, and therefore 1 metre in RL doesn't necessarily correspond to 1 metre in SL (sadly). My RL height is 1,75m, my SL height is some 2m and still I'm shorter than almost everyone I know. It is how it is…
The face on the other side is somewhat of a show for maturity, though between 15 and 20 the change of an average RL person is very little, it's not like 20 and 40. Of course I agree with the snot comment (lol!), but my point is that other than such obvious signs, it's difficult to tell someone's apparent age.