Crazy Desire (1962) on Netflix

A middle-aged and slightly conservative businessman meets a band of rowdy youths and is smitten by one named Francesca. He is pursuaded to join their party and even pay for most of it, as he finds himself strangely attracted to Francesca, who seems his complete opposite.

—Murray Chapman <muzzle@cs.uq.oz.au>

Antonio, a divorced, middle aged, uptight engineer/businessman, goes on a road trip and meets fickle teenager Francesca with her group of friends on summer vacation along the way. She flirts with him for kicks and gets him to fall for and obsess with her.

—fellini813

Antonio Berlinghieri is separated from his wife, has a lover and a son whom he keeps in boarding school in Pisa. Just as he is going there to meet him, he runs into a group of boys heading for the sea. At first annoyed by their jokes, he then lets himself be overwhelmed by the desire to return to being twenty years old and by their joy and falls in love with Francesca and her vitality. Go with them to a picnic on the beach and celebrate and celebrate and celebrate - But the years are no longer twenty..

—Baldinotto da Pistoia