He was originally written as a 16 year old. When Winkler was cast (27) it was generally accepted as somewhere between 18-20
Even though in real life the actor is almost a decade older than the other actors, in TV world he’s supposed to be only a few years older than Richie.
He’s a high school drop out (and “bad kid”) who befriends Richie when Richie tries to help him go back to school. He ends up dropping out again, but stays friends with Richie. He works as a mechanic, eventually owns his own garage and is part owner of the diner.