Every-single-day, we hear how Leborn is dominating the leauge at the ripe age of 38. Absolutely showing no signs of slowing down and probaby will continue to do so for a while.
But if you notice carefully, there are similar trajectories with other teams and players as well.
We see how Curry has been putting performance better than his MVP season. How durant is still a freak at 35, complete bucket.
So these players will mostly, follow the same form and pattern as Bron and will continue to dominate even at their 40s. And we will see a wave of players after than who will reach their primes and prolounge it by different attributes and play at the highest level at older ages.
Now this theory goes totally against the injury ridden NBA of today, so that's something one can't really predict, thus my guess is that players who don't suffer carrer ending injuries and who style thier play intelligently, knowing they can't red line 95% of time, they will have the breakthrough and carrer advancement similar to L James.
Finnaly, this is the trend we saw with Jordan as well. He came, and turned into a Superstar, now every team has a Superstar and the leauge is not very Superstar heavy. Most of the players are adequately skilled and you won't find that much parity in players compared to the 80s and 90s.
Just like today the players are learning to player longer (Lebron), they already leaned how to play better (Jordan).