LeBron James, entering his 21st NBA season, will turn 39 in December and on many nights will play against opponents about half his age, while trying to lead the Los Angeles Lakers to their 18th championship in franchise history.

“I think it all starts with the mind, obviously,’’ James said Monday on media day during an interview with Spectrum SportsNet.

James looked as muscular as ever, capable of bulling his way to the basket, or throwing down a reverse jam. But he made it clear that his path to four NBA championships and the league’s all-time leading scoring record does not start with the body.

“You know, the mind controls everything,’’ he continued during his interview. “I think being able to keep my mind fresh, so just … allows my body to kind of follow after that. Then able to continue to step into the gym and continue to prepare myself for another season. I felt like I had a heck of a summer.’’

Perhaps the most encouraging development from the summer: recovering from a foot injury that at one point sidelined him for 13 consecutive games.

“Was able to get my foot back to where I was before the injury last year,’’ James said. “Was able to recalibrate, get my mind back as far as my game and things of that nature. So feeling pretty good right now going into training camp.’’

Why did LeBron James come back this year?

James said he had “a lot to think about’’ after the Lakers got swept by the Denver Nuggets in the Western Conference Finals. He later told ESPN he would contemplate retirement.

But in July, at the ESPY Awards, he announced he was coming back. And on Monday, he some insight about his decision.

He said watching his sons, 18-year-old Bronny and 16-year-old Bryce, play basketball inspired him. Then there was the work that Lakers general manager Rob Pelinka and coach Darvin Ham did with the Lakers roster.

“To be honest, if I want to keep it all the way real, the job that Coach Ham and Rob did with acquisitions that we got coming in, that kind of inspired me too,” James said. “I feel like we got a really good team. They did a great job this summer being able to re-sign some of the guys we already had here with the camaraderie.’’

The returning core, led by James, also includes Anthony Davis, D’Angelo Russell, Austin Reaves, Rui Hachimura. New acquisitions include long-range shooters Gabe Vincent and Cam Reddish, forward Taurean Prince and center Christian Wood.

How good could the Lakers be?

To get back to the NBA Finals, the Lakers likely will have to get past the team that stopped them from getting their last season. Appropriately enough, the Lakers will open the regular season Oct. 24 against the Nuggets in Denver.

“Damn sure not satisfied with the way things ended last year,” James said, “but very optimistic about where we can carry on going into the season.”