How old was The Fonz?

"Happy Day's" storyline began around 1955 or 1956; at that time, most of the central characters (Richie, Potsie, Ralph, in particular) were around 17 I think. Along comes Arthur Fonzarelli, mechanic, former gang member, high school dropout. There is an implicit recognition that he is quite a bit more experienced in the ways of the world then his friends, that he is probably some years older than they are. How much older, though? I don't believe Fonzie's true age is ever revealed, but I could be wrong.