It used to be 20. 70s—>50s (American Graffiti, Grease, Happy Days), 80s—>60s (Summer of Love, Touch of Gray, Love Shack), 90s—>70s (Tarantino, lowrise bell bottoms), 00–>80s… When you’re 18-25, your model is the previous generation. Around 30, the pattern resolves and your grandparents somehow become sophisticated in hindsight.