American Graffiti actually took place in 1962, and was only tens year out. But since it was also pre Kennedy assassination, pre Beatles/Brit invasion, pre Gulf of Tonkin era, it was easy to conceptually view it as more emblematic of the previous decade than what sticks out mostly as "the Sixties" in people's minds.

Fifties's nostalgia in the Seventies included two top ten comedy shows for most of that decade (Happy Day's and the Korean War set M.A.S.H.) both of which were either off the air by the mid Eighties or had long abandoned any real pretense at period authenticity.

From my view, the Fifties nostalgia wave peaked in 1978 with the theatrical release of Grease as well as the Buddy Holly Story. Sha-Na-Na, had a weekly TV show and probably saw it's highest ratings at that point.
By the next year, the focus would be on the coming of a new decade and trends like punk and New Wave were filtering into the mainstream with groups like Blondie.
By the end of '81, Sha-Na-Na was off the air and the biggest influencer of fashion and trends for that decade and beyond was just dawning (MTV). Fifties era iconography was definitely passe by '82-'83. When I went to see The Big Chill, no longer were the adults in the picture listening to do-wop or reminiscing to Fats Domino on the soundtrack- it was all Sixties.