hormones

It's the hormones I think and a combination of the fact that they're more full of energy at that age. The glow you talk of is very real. Blood is pumping around the body much more efficiently leading to sometimes reddening of the cheeks. The skin is in much better condition than after age 20 too. Of course there are some that are susceptible to spots but generally speaking, young skin has more glow.

Young people are more light-hearted and optimistic too. If you have ever worked with young teens (I have), then you will know that they can laugh at virtually anything. Given the choice, I'd prefer something much younger, but I like an adolescent too.