Remarkably, this doesn't appear to be a "career bonus" solely for (the less injury-prone?) point guards or centers. All positions are present, including power forwards like Karl Malone.
I'm not in the US -- could one say that current NBA games have a considerably faster pace (more jump-and-shoot) as compared to, say, the early 1990s? This would probably require more physical strength, which makes the 40+ guys even more interesting.
I played handball during my teens, and I remember a renowned goalkeeper who performed incredibly at age 45+ (possibly even at national team level or thereabouts; at least as first goalie of our country's number one team). IIRC, he was also a dedicated smoker, always having a pre-game cigarette, LOL -- in all seriousness, this probably contributed somewhat to keeping him exceptionally alert during the game.