WALTHAM, Mass. – A year ago, James Young arrived in Boston as an 18-year-old kid.

Now, he’s slowly turning himself into a man, both physically and mentally.

A visibly bulked up Young appeared in front of the media Wednesday afternoon following Boston’s first summer league practice. He confidently informed the reporters who circled around him that his appearance was no mirage.

“When I first got here [after last season] I was 200 (pounds), or 195,” Young said.

“I’m up to 220 now.”

Young’s body is clearly developing, and as Celtics summer league head coach Jay Larranaga stated, so is the shooting guard’s mind.

“He’s just maturing as a person more than a player or anything else,” Larranaga said. “You get more comfortable in your surroundings, you start to kind of figure out how things work. I think his biggest growth is off the court and just his approach.”

This is music to Boston’s ears.

There has never been a question about Young’s talent, skill or promise. He’s blessed with high-level potential.

There have, however, been questions about his work ethic. This is his first major step toward silencing those questions.

No longer do the Celtics need to wonder about whether he’ll show up to the weight room on an off day. No longer do they need to worry about what he’s doing off the court. He has found himself and is turning into a reliable prospect.

“I felt like it was more mental for me,” Young says of his newfound approach, “because confidence-wise I was up and down the whole season.

“Now I’m not worried. I’m staying off of social media and stuff like that. I’m just worrying about me and getting better.”

The Celtics are already seeing the dividends. Young says his added weight is allowing him to do things on the court that he had never done before.

Asked how his strength has affected his game, Young explained, “Definitely attacking the basket. I use my body a lot more in drawing contact and the coaches have really been seeing it.”

This is an area of the game that, according to Larranaga, head coach Brad Stevens emphasizes to every player in the program.

“Coach Stevens talks a lot about owning your space, whether that’s at the offensive or defensive end,” Larranaga said. “That’s a big deal – being able to offensively to not get pushed off your cuts, to not get pushed off a screen, and defensively to push a man off their cuts or their screens.”

At 220 pounds – the same weight Marcus Smart was listed at last season – Young is now equipped to own his space quite well at the NBA level.

Additionally, the 19-year-old has been working on his game at the defensive end of the court. He dedicated two to three straight weeks of workouts to honing his defensive slides, communication and help defense. That’s what he wants to showcase more than anything during the Salt Lake City and Las Vegas summer leagues.

“(I want to prove) that I can play at both ends of the floor, not just play offense,” he emphatically stated. “That’s what I’ve been working on a lot more – defense – so I want to prove that to them.”

If he does, the entire NBA will soon begin to understand why the Celtics invested their 17th overall pick in Young one year ago.

There’s no question that Young is trending in the right direction. His body and mind are maturing, and now it’s time for him to prove his worth to Stevens and the Celtics.