Not only them. The Bucks are in the playoffs already, 76ers' average age is probably 15 or something and Detroit also has some young talents. Most superstars are young anyway.
LeBron is in his prime but Curry, KD, Harden, Lillard (so damn underrated, most clutch player since MJ imo), Davis, Kyrie, Wall, Westbrook etc.
The performances of older players like Duncan are often overlooked because they do not put up freaky numbers anymore but come play-off time they still pretty much rule the league. But considering popularity and looking at the outstanding players of most teams, they are damn young.