That's really kind of a myth, particularly for an "old" rookie like Jason. to whatever degree it would be true for bigs, it would be for 19 year old kids coming in without the bodies to compete with grown men. JT came into the league at 22, is moving toward 25 now. He's full grown. Guys his age used to routinely step right into the league and dominate if they were able to back in the old 4yr college days, and even the slow developers might follow the normal third year rule at best. Very few guys improve by any great leaps after 25, big men or not.
JTs bigges thope has always been thst so many of his errors are essentially mental, adn that's one area of the game where you do keep on getting better. But he's going to have to show that on the court fairly quickly, because we've got to many options now to be coddling hiim. Two games is obviously a ridiculous time period for decisions, but three months and approaching the trade deadline could get interesting, adn Jason would be well advised to not make it interesting for himself.