In real life, Henry Winkler (Fonzie) is 8 years older than Ron Howard (Richie Cunningham). Did that age difference transfer to the characters?

Fonzie definitely seemed older than the kids. Without any research, I would guess Fonzie was at least 5 years older. But could have even been 10 years older.

In season 1, there is an episode where Fonzie goes back to school. So he is a few years older, as he says the girls there now would not have seen his tatoo, but not old enough that they don't let him back into school. Not sure what the age limit would have been back then.

Fonzie's only about a year or two older than Richie and friends. In season three, they celebrate his 20th birthday, which would mean that Fonzie was about 17 when he met Richie in the flashback.