What did Kenny “The Snake” Stabler look like at 38 years young?

It was the smoking.

It wasn’t just if you smoked or not, everywhere you went indoors you were inhaling cigarette smoke and just existing in a cloud of smoke … usually from the moment you got home from the hospital as a baby.

Add in the greater awareness of the effects of the sun along with the normalized use of sunscreen and most people aren’t prematurely aging their skin/hair.
In addition to smoking, I think they were more likely to drink hard liquor on a daily basis. I think that also ages a person.