WALTHAM — Avery Bradley is a particularly weathered, surgically scarred 23.

The Celtics guard is starting his fifth NBA season and is entitled to the double take he made upon reading James Young’s birth year.

“He was born in 1995,” said the disbelieving Bradley, who almost comes from another decade — 1990.

Young, whose last name supplies an unfortunate hook for those who have fun with his age, laughs it all off.

“I’m a youngster. People call me a baby to joke around, but I’m used to it,” he said after yesterday’s opening session of training camp.

Unless the Celtics draft another 19-year-old next year (Young was the No. 17 pick in June), the joke will continue for someone who always was one of the youngest in his class, and last fall the youngest player in Kentucky’s absurdly talented class of freshmen.

And now he’s the youngest player here. Something of a mystery, too. He missed the Orlando Summer League while recovering from a concussion that resulted from a minor car accident last spring.

Some don’t know what to make of him now, but Young also is accustomed to that reaction.

“A lot of people are sleeping on me anyway, but I’m not even thinking about that,” he said, not seeming to mind.

“It’s been happening all my life,” Young explained. “When I went to Kentucky a lot of people said I wasn’t going to start. That gave me a little chip on my shoulder. On draft night, they said I was going to go a lot later in the draft.”

But the Celtics need scoring, and Young has the kind of perimeter shot that demands attention. The outside world, and especially Celtics Nation, might not know a lot about the 6-foot-6 swingman, but one person who seems to already know Young very well is his new coach.

“I have a …read more