Willie Nelson 'Never Thought' He'd Get to Age 90 — but Says the Milestone 'Ain't Nothing' (Exclusive)

It's not every day a music legend turns 90.

So when it came time for Willie Nelson to celebrate on April 29, his famous friends — including fellow Highwayman Kris Kristofferson, Sheryl Crow, George Strait and Miranda Lambert — went big, gathering for a two-day concert extravaganza, Blackbird Presents' Long Story Short: Willie Nelson 90, at the Hollywood Bowl.

"I never thought I'd get here," Nelson told PEOPLE ahead of the festivities, which drew an estimated 18,000 fans each night.

Still, in signature Nelson fashion, he waved off the fuss over his milestone birthday: "This ain't nothing. It's another day."

The best gift Nelson could receive after more than 60 years in the business was just a simple good time onstage.

"I get a lot of fun out of playing for an audience," he says. "There's a great energy exchange there. It's what keeps me going."

Despite Nelson's reticence, those who have drawn inspiration from his genre-pushing music and social activism were eager to celebrate his impact.

"You can't talk about American music as a whole without Willie Nelson," singer Charley Crockett told PEOPLE backstage. "That's why this is a cultural event."

Born during the Great Depression and raised by his grandparents in Abbott, Texas, Nelson wrote his first song by age 7 and joined his first band at 10. By 1960 he took his music dreams to Nashville, where he broke through with his debut LP, ... And Then I Wrote, two years later.

Though he had written hit songs through the '60s, Nelson grew tired of the Music City scene in the early '70s and moved to Austin, where he helped pioneer outlaw country with albums such as Shotgun Willie and Phases and Stages.

In 1979 he kick-started his acting career in The Electric Horseman, his first of more than 30 feature films. And the next year he released his quintessential hit "On the Road Again," an ode to his nomadic lifestyle that still rings true.

"I quit after every tour, then two days later I'm ready to go back," he says. "Billy Joe Shaver wrote in a song, 'Moving is the closest thing to being free,' and that's the way I look at it. I enjoy riding up and down the highway."

While Nelson still loves to hit the road, he's taken a break from songwriting.

"Roger Miller told me, 'Sometimes the well dries up, and you have to wait and let it fill up again,'" he says. "I believe that."

Luckily Nelson has 73 solo studio albums — plus three with his Highwaymen bandmates Johnny Cash, Waylon Jennings and Kristofferson — to fall back on.

These days the longtime cannabis connoisseur also no longer smokes or drinks. "That's added a few days to my life, I'm sure," Nelson says.

He's feeling the effects of aging — his hearing's "not the best," and he can only do most of his martial arts routines in his "mind" now — but Nelson still feels young at heart.

"As they say, laughter's the best medicine," he says. "I've always enjoyed a good joke."

That might be the key then to his 31-year marriage with fourth wife Annie D'Angelo, 66.

"I call her my pet rattler," he quips of D'Angelo, with whom he shares sons Lukas, 34, and Micah, 33. (He's also dad to Lana, 69, Susie, 66, Paula, 53, and Amy, 49, from previous relationships, as well as his late children Renee and Billy.) "She's my lover, my wife, nurse, doctor, bodyguard."

As he enters a new decade, retirement is far from his mind — but he's finally slowing his pace.

"There's probably other things I will do and can do, but I'm not going to push myself too hard," says Nelson, who's being inducted into the Rock & Roll Hall of Fame in November after touring through the fall. "I know one day it all ends, but I'm not rushing it."