Two possibilities that are sort of similar.
Lebron could be 40 years old and he'd still be as athletic as 1990s Magic...who was good enough to lead a team to the Finals.
I think if he's smart though, he takes a role pretty much like the one that Draymond Green has on offense, but his passing and scoring skills are both well above Green's so he'd do it better. Be the forward that the ball rotates around in an offense with a lot of passing, without him having to drive on every possession, and he could easily rack up 20-12-8 numbers in his late 30s without having to expend a ton of energy on that end.
The greats like MJ and Kobe did this and relied on their post game. Most older players use their post game and pull up game as they get older because they fatigue easily.
At 30 MJ began to use more of a drive and pull from 18 feet and a post game to reduce the wear and tear. Then expanded this even more once he came back from the first retirement.
Lebron problem is he doesn't have a great facilitator on the team such as a pippen and Lamar odom to pass him the ball in the post or run the offense. This is what MJ and Kobe had (not to mention an offense that is setup for post players in the Triangle).
The cavs are god awful post passers outside of Wade, Lebron and maybe crowder. Hell anyone that has watched the Cavs 3 years prior to this year knows that delly was the best post passer on the team outside of lebron.
The cavs would basically run post ups at the 4 for Lebron with delly the passer and pick and rolls with Delly and Lebron.
So playing in the post hasn't been an issue for lebron for years. It is just he needs someone who can pass the ball to him in the post (cause backing down from the perimeter is ineffective). Plus a point guard that can run the offense so he doesn't have to do that at all time. Just too much to ask a man to run the offense, be the leading scorer, create for others, play in the post and everything else Lebron is asked to do.