I believe he was supposed to be just slightly older than Richie and the gang, at least originally. Mr. & Mrs. Cunningham essentially become his adoptive parents and treated him like a surrogate son. I believe he was intended to be in his late teens at the start, having graduated, or dropped out late in his high school career, and then went to work instead of continuing his education--a fact that somehow didn't stop him from being a teacher later in the series. In the early episodes, Mr. Cunningham's interactions with him display far more parental concern than they would be later in the series, but by that time the Fonz was a massive cultural phenomenon (truly unlike anything you see nowadays--this coming from someone who had Fonzie t-shirts, posters, a lunch box, record albums and other items), which completely changed the direction and makeup of the show.
Bottom line is that he wasn't a creeper. It was a show in the 70s portraying life in the 50s, with a character who was supposed to be 18-19 years old hanging out with 16-17 year olds. Plus, for several years he was literally the coolest human being on the planet.
Well believe or not . I still have my full size standing wall plug in Fonz pinball machine. It's made by coleco and is free standing about 5 foot tall to top of the back of machine. And still works great too.