Ever since the mythic ’60s ended, countercultural idealists have been grappling with the loss of what they believed would be their eternal youth. Considering how many serious books and movies have addressed the aging of the Woodstock generation, it’s surprising to find Flashback, a seemingly insubstantial film that has something important to say on the subject. [...] Screenwriter David Loughery cleverly builds this simple role reversal into an affecting vehicle for exploring identity and growth. Against all odds, the movie manages to avoid easy caricatures. [...] Unlike many such voyages through the past, this is one ’60s trip that’s worth repeating.