As LeBron James warmed up before a recent game against San Antonio, he began joking about his longevity.

“I’m gonna play until I’m 40,” he said, taking a shot. “Then, after 40, I’m gonna go all left-hand until I’m 45. I’m gonna score 5,000 points with my left hand. Then I’ll be done.”

James was smiling. His words were outlandish. But it’s not impossible to think if he wanted to do those things, he could.

As James celebrates his 39th birthday on Saturday amid his 21st season in the NBA, he’s doing things that are completely unprecedented for someone in this stage of his career.

He’s averaging 25.4 points on 53.9 percent shooting from the field and a career-high 41.3 percent from beyond the arc, along with 7.8 rebounds and 7.3 assists a game. No player has ever averaged more than 7.4 points in their 21st season. Not to mention, he already holds the record for most points averaged for someone in their 18th, 19th and 20th seasons.

This season, he is the oldest player in the league – and the first to hold that title and be in the top 10 for preseason MVP odds.

While fans and pundits will continue the never-ending debate over who is the greatest player of all-time, two-time league MVP Giannis Antetokounmpo believes there’s no contest.

“I say [LeBron] is No. 1,” the Milwaukee Bucks superstar said earlier this season. “And he’s still going. He hasn’t slowed down.

“…He sets up the blueprint for all of the athletes in the NBA to follow. For 21 years, healthy, good, consistent. NBA Finals 10 times. …He’s incredible. That’s the blueprint that I’m going to try to follow, try to mimic. I want to be 39 years old and be able to keep on playing and the next guy talks about me.”

Video:

James is currently the oldest player in the NBA. His longevity is so stunning that he has spanned enough decades to play against nine fathers and their sons. The average NBA player is 26 years old, closer in age to James’ 19-year-old son Bronny than him.

But James hasn’t lasted this long as a bench ornament or a savvy veteran whose role is to advise the youngsters. Rather, James is still the best player on the court many nights.

James holds the record for the most 30-point games after turning 35 (89). Last season, he became the oldest player to score 30 points in seven straight games. He also became the all-time leading scorer in NBA history last February – and he’s currently on pace to reach 40,000 career points, a number that was long deemed impossible.

Just last week, James had 40 points, seven rebounds and seven assists in a win over Oklahoma City, while shooting 65 percent from the field and 5-for-5 from beyond the arc.

What he’s doing is so mind-bogglingly stunning yet so normal for him that some of the league’s biggest stars think that it will take a while for the enormity of it to sink in.

“When you think about him being a teenager, and now he’s close to 40 – and he’s been at the top of his game since,” Bucks star Damian Lillard said.  “…I think people will look back at that body of work over 20 years, or however long it’s been, and when it’s over with, I think they’ll have more respect for it.”

Most of the league’s superstars grew up idolizing James. And to this day, he remains their measuring stick.

Spurs rookie sensation Victor Wembanyama wasn’t even born when James was selected as the No. 1 draft pick by his hometown Cleveland Cavaliers in 2003. Twenty years later, as the 19-year-old Wembanyama was preparing to enter the league, he said on “The Old Man & The Three” podcast that he was most looking forward to sharing the court with James but “to play against LeBron is going to feel so weird.”

The league’s more established stars remember feeling similarly.

“He’s like a barometer,” Phoenix Suns star Kevin Durant said. “Just look at his box scores after my games a lot – my whole career. You see what he did. Watching highlights since I was in high school.”

For James, what he’s accomplishing at this age is no accident.

He has iced after games since he was in high school. He tries to get at least 12 hours of sleep a night. He meditates. Regardless of what time he lands in a city, he’ll do all of his activation exercises. He studies the game and his opponents. He constantly tweaks his play to remain a few steps ahead.

He also purposefully enters every season with something to prove to himself. And it’s clear that he has embraced the narrative that he’s over the hill as a motivational tool.

Going back the past few seasons, he’ll often use the hashtag “WashedKing” when he reposts his stats or videos on social media. He has appeared in Nike commercials where he literally plays one-on-one against Father Time and, of course, wins.

And let’s not forget what happened last postseason when former Memphis Grizzlies foil Dillon Brooks called James “old” and said he’d only respect him if he scored 40 points against him. James responded with a 22-point and 20-rebound performance in Game 4 of that series – the oldest player to put up those stats.

James may be ancient in basketball years. But when asked if he feels his age, he didn’t hesitate.

“I don’t,” he said. “I actually don’t. I’m just going out there and just playing free, free basketball. I feel like there’s not anything that I cannot do that I did in my 20s out on the basketball floor.

“So just continue to try to be the ultimate triple-threat, you know, scoring in the low-post, scoring in the paint, scoring in midrange, scoring 3s, getting to the free-throw line when I can, things of that nature. Always trying to keep the defense at bay. And I always have the ability to be able to rebound and find my teammates.”

Even though James hasn’t really shown any visible signs of physical decline, it’s no secret that what we’re witnessing is ephemeral, like a meteor shower that gets brighter and brighter before suddenly disappearing.

Last season, there were multiple times when James would sit in the locker room and talk about the exhausting travel schedule. And after the Lakers were swept by the Denver Nuggets in the Western Conference finals, he intimated that he was considering retirement, saying, “I’ve got a lot to think about.”

But he has long said that he wants to remain in the NBA long enough to become the first player to play alongside his son. So, if the basketball world is lucky, we’ll get to witness his greatness for at least one more season. (Bronny is currently a freshman at USC and could become eligible for the NBA Draft this summer.)

In the meantime, to watch James is to seemingly watch the laws of nature be defied.

He has been the face of the league since he was a teenager. And even though that face has aged and grayed a bit, two decades later it remains the epicenter of the basketball universe.

Melissa Rohlin is an NBA writer for FOX Sports. She previously covered the league for Sports Illustrated, the Los Angeles Times, the Bay Area News Group and the San Antonio Express-News. Follow her on Twitter @melissarohlin.