It’s said that age is just a number.

That’s never more true than when you become that number.

When I was a lad — we’re talking pre-cable TV, 8-track, death to disco days — quarterbacks were done by their mid-30s. Well done. It wasn’t uncommon to see Joe Namath drop back to pass and lose an elbow or some other key body part. Johnny Unitas played until he was 40, but by then NFL Films no longer had to play his highlights in slow motion.

These days, the Three Wise Men of NFL quarterbacking fame — Drew Brees, Peyton Manning and Tom Brady — are turning back the clock on Father Time.

In fact, they’ve called him to the coach’s office and told him to bring his playbook.

Brees, 35, is seriously talking about playing another 10 years.

“In my mind, I’m not in my mid-30s,” the Saints hallowed quarterback said earlier in training camp. “I’m 25.”

Brady is 36 (he turns 37 Sunday) but speaks not of retiring but like he’s intent on retiring his current contract with the New England Patriots when it ends in 2017. “I want to do this for this team as long as I possibly can,” he said.

In Denver the 38-year-old Manning not only recovered from career-threatening neck surgery but hit another gear as he threw for nearly 5,500 yards and 55 touchdowns in 2013.

“He’s playing at the highest level I’ve ever seen him play,” said Broncos tight end Jacob Tamme, a teammate with Manning in Denver and Indianapolis.

How are they doing it?

Certainly the pass-friendly rules changes help. It would be better for an NFL defender to sell government secrets to the Chinese these days than hit a quarterback too late or too high — or low.

Great surgical techniques not available to Broadway Joe or Johnny U. have also prolonged careers.

While Manning came back from neck surgery, Brady rebounded from a serious knee injury in 2008 and Brees famously had his career rescued by extensive shoulder surgery (performed by LSU grad Dr. James Andrews, who was here over the weekend) before landing in New Orleans in 2006.

But it takes more than that to keep playing well past the established prime. For Brees, it starts with an intense regimen of cross training, a missionary zeal when it comes to what he eats and a zealous defense of his down time.

“Has my training changed since I first came into the league?” Brees asked. “Absolutely. I focus solely on cross-specific training. If it’s not relevant to what I’m doing on the field it’s a waste of time.

“Rest, diet, sleep habits, they’re all a factor and will probably become more important.”

Brees’ devotion to his diet has become the food stuff of legend in Saints circles.

“If you just want to get his attention you just say, ‘Hey they have gluten-free pancakes today,’ ” Saints coach Sean Payton said. “And he would be like, ‘Where?’ ”

Brees is apparently astute enough to avoid the poppy seed muffins.

“On Friday I said I thought I play til age 45,” Brees tweeted Sunday. “I have been ‘randomly’ selected for drug tests the last 2 days. What’s up with that! Lol”

All kidding aside, and despite the fact he could be voted pope of New Orleans right now, Brees knows he plays an unforgiving game.

“I’m trying to play like I did when I was a kid, having fun throwing the ball around with my brother,” he said. “But I know it’s a serious business. They’re not going to keep you if you can’t play well.”

But whose to say Brees isn’t playing as well as ever? He’s coming off his third straight 5,000-yard season in 2013. Can he still be throwing for five grand when he’s 40?

“I’m serious,” Brees said. “But I’m not delusional. I know that’s something that will be extremely difficult to do.”

Difficult, but hardly impossible. Eighteen quarterbacks in NFL history played into their 40s, including former Saints quarterback Wade Wilson (with Oakland in 1999), Hall of Famer George Blanda (though he really stopped being a quarterback at 39) and Doug Flutie.

“It could be done,” Brees said. “A lot of things have to fall into place.

“Through your career you hit a lot of milestones. I was drafted in the second round by the San Diego Chargers and I started as a backup to Doug Flutie (in 2001), so you think, ‘OK, my goal is one day to be a starter.’ Then you become a starter and you say, ‘My goal is to make it to the Pro Bowl.’ Then I thought about playing double digit (seasons). Then to playing until I’m 35. So what’s the next thing?

“I know it’s one year at a time. But why not push the envelope a bit?”

There’s pushing the envelope, then there’s inventing e-mail.

Brees, Manning and Brady have a message for Father Time: Don’t wait up. We’re going to be here awhile.

Follow Scott Rabalais on Twitter: @RabalaisAdv.