By Matthew Geagan, CBS Boston

BOSTON (CBS) — Jayson Tatum is growing in front of us all. Just four years ago, he was a 19-year-old, fresh-faced pup looking to break into the NBA. Fast forward to 2020, and he’s a 19-year-old superstar-in-the-making.

Except that Tatum isn’t 19 years old anymore, no matter how much Celtics fans want for Tatum to remain a teenager. It’s a point that Tatum — now an All-Star and future max contract player — has hammered home a few times as he prepare for his fourth NBA season.

He certainly doesn’t look like a 19-year-old, not with the new chiseled physique that he’s sporting. The now 22-year-old Tatum must have been a mad man in the weight room during his quick two-month offseason, with another loss in the Eastern Conference Finals all the motivation that he needed.

But as a more mature, grown-up looking Tatum explained Wednesday, his bulky arms are not just the product of hours and hours of lifting.

“I’ve gained probably about 10 pounds,” Tatum told reporters on his Wednesday afternoon Zoom session. “Been in the weight room, but I’m getting older and my body is starting to mature a little more.

“I’m not 19 anymore,” he added.

There it is again, that reminder that Tatum is not calling Neverland his home during the offseason. He does indeed get older, and his body is indeed changing.

Tatum is heading into the 2020-21 season a bit bigger than he was last season, both in height and in weight. If he was considered a “problem” by LeBron James and other NBA stars last year, just imagine what Tatum has in store for them this season.

Tatum’s maturity is not just physical, either. He elevated his game to superstar status for large stretches during last season, and many are already putting him in that stratosphere of NBA talent. But Tatum wasn’t ready to play that game on Wednesday.

He said that ranking players and elevating an All-Star to a superstar is something for the pundits. He’s not focused on any of that, and just wants to be the best all-around player that he can be for the Boston Celtics.

“I guess that is for other people to decide if you are or aren’t. I don’t get caught up into the technicality of being a superstar. Every year, I want to improve and get better,” he said. “I don’t put a ceiling on it; I want to be one of the best. I want to be one of the best to play this game. I don’t think that, now, I feel different. Every year I take the approach that I want to be better than I was last year.”

Superstar or not, Tatum is the leader and the face of the Boston Celtics. As the top player on the team, he knows that his job is not only to score a lot of points and lead the team to victories, but to make everyone around him better.

“I think it just has to elevate my play-making ability,” he said of his status on the team’s pecking order. “I felt like it was trending that way during the playoffs, when I saw a lot more double teams and blitzing, with my ability to make plays and make the game easier for the guys around me. So, just be more of a play-maker and get other guys involved.”

Tatum’s physicals changes are easy to spot, and we can easily see the effects they have on his play. Being a little bit taller and a little bulkier is going to help him clear a path on his way to the rim, and he’ll probably have some more thunderous finishes thanks to those two tickets to the gun show. That is all part of the maturing process for someone who enters the NBA as a teenager.

His mental maturation is just as prevalent though, albeit a little harder to see with the naked eye. If Tatum is at the point where he’s not only elevating his own game but making life easier for everyone around him, then the Celtics have a truly special players on their hands.