Paul Pierce was 20 years old when he was drafted by the Celtics in 1998. Eleven years later, he still doesn’t feel a day out of college.

He may not be in Kansas any more ...

“I still feel in my early 20s,” he told WEEI.com after a recent practice. “I feel like I’m back in Kansas.”

Pierce turns 32 on Tuesday, and a new lifestyle has him energized as he enters his 12th NBA season. Last month he was one of several Celtics who showed up to media day with a noticeably slimmer frame. While Kendrick Perkins and Glen Davis admitted to dropping up to 15 and 30 pounds respectively, Pierce (who is listed at 235 pounds) isn’t paying attention to the scale.

“I maybe lost two pounds, I don’t know,” he said. As for losing body fat, “Probably so, I haven’t measured it, but most likely I have.”

Pierce isn’t counting the pounds because it’s how his body feels, not what the scale reads, that matters more to him. And even though he feels young, he understands the reality of being a professional athlete in his 30s. Pierce made a commitment to improve his overall health and kick the habits that could eventually catch up to him.

“I just think it’s more important because the body is aging, it’s getting older,” he said. “You’re not as quick as you used to be and your body has to adjust with the times. I mean, what I used to do when I was younger are some of the things I can’t do now. I’m stressing more rest, I didn’t sleep as much when I was younger, so a lot of things change. It’s easier to gain weight the older you get so that’s why I decided to change a lot of my habits.”

The change in his physique has more to do with the changes he endorses through his healthy living initiatives rather than a regimented weight loss program.

... but Pierce still feels young.

“I think it has more to do with my eating habits,” he said. “Ever since I changed everything I’m doing as far as my foundation with The Truth on Health and FitClub34, I’ve got to practice what I preach. I’ve pretty much benefited from the things I’m doing of that nature.”

He added: “I’ve just changed up my eating habits as far as eating a lot of fish, chicken, vegetables, not as much red meat — almost no red meat — no sodas, not a lot of juice. I drink a lot of water and Gatorade and Powerade.”

Ray Allen, a stickler for healthy living, already has noticed the changes in Pierce. Allen was 32 when he was traded to the Celtics in 2008, a time when he says he began to get in his best shape by eating better and getting more sleep. He understands firsthand the importance of Pierce taking care of his body at this pivotal stage in his career.

“Just yesterday in the weight room I told him he was starting to look skinny,” Allen said. “You know, Paul has always had like a heavy look, a heavy body. Now you see in his legs he’s slimming down, his upper body is slimming down. I think it’s the natural evolution when you get into your 30s and your metabolism starts to slow down and you start thinking about eating better, getting more sleep, and then your body starts to show it. And then as for us, this schedule is so demanding you really have no choice, your body will tell you you need more rest.”

Pierce has learned to listen to his body, and because of this he can celebrate his 32nd birthday feeling like he’s in his 20s.