It’s not something inherent to music or musical creativity. Classical music follows the same pattern as writing, with the greatest work produced mid to late car

It’s not something inherent to music or musical creativity. Classical music follows the same pattern as writing, with the greatest work produced mid to late career. Beethoven, Brahms, Wagner, Bach, take your pick. So it’s probably just explained by the youth culture around popular music and bands, as other commenters mention.