Frankie Valli on Turning 89, Frank Sinatra and Nick Jonas Playing Him in 'Jersey Boys' Musical (Exclusive)

"[Age is] really a frame of mind. It really is. There are wonderful things you learn at every age as you go along," Valli tells PEOPLE exclusively

With a heart of gold and spirit of a 20-year-old, Frankie Valli is celebrating 89 years around the sun — and still getting the most out of life.

In light of his birthday on Wednesday, the legendary singer spoke to PEOPLE about feeling young, his friendship with Frank Sinatra and much more.

"It feels pretty good," the Four Seasons lead singer tells PEOPLE of turning 89.

"It's just another year, the way I look at it. So far, knock on wood, my health has been good enough so that I can drive — I have all my marbles and I'm always interested in learning," he continues. "I like reading scripts and I always go over new music and listen to other people's music for inspiration."

When asked what keeps him young, Valli — who says he enjoys listening to rap music these days — claims he "eats baby food."

Related:Frankie Valli and Musical Star Granddaughter Olivia on Their Shared Love of Performing — and Pasta

"I am joking! Your mind is what keeps you young — you have to keep using it. I know people who couldn't wait to retire, and when they got to the moment of retirement, they started getting old real quick," he says. "It's like they had too much on their mind, and if they stayed — even in a part-time way — a little busier, they could've stretched out those extra years that we all look for."

Valli adds, "[Age is] really a frame of mind. It really is. There are wonderful things you learn at every age as you go along. You can always learn something, even from kids, if you pay attention to what's going on."

Reflecting on the thought of retiring, the "Sherry" singer says that he doesn't plan to stop anytime soon.

"I'll probably tour until I can't do it anymore, but even if it's a limited amount of touring, it's something I love and I love people. If it weren't for people, I would not have had a career in the music business," he says. "It;s those guys that really went out and bought the product as you made it, loved you and came to see you. When you go to a different city and you work in a different venue, it's like visiting friends. That's the best way that I can put it."

"It's a wonderful comfort zone. It really is, and it gives you what you need when you need, when there are things on your mind and so forth and so on," he continues. "When you go out and perform, you have this wonderful audience and you forget about all of your troubles and problems. They all go away."

Meanwhile, Nick Jonas is set to portray Valli in the upcoming Jersey Boys movie musical — and the singer shared his thoughts on the actor.

"Well, [from] what I've seen of it so far, I think he's brilliant. The Jonas Brothers are terrific to start off, and they also come from New Jersey," he says.

Valli — who had a say in choosing the star — also tells PEOPLE that he was "very happy" Jonas is filling the role.

Last week, a street in Newark, New Jersey — where Valli grew up in a housing project — was renamed after him.

"It was quite exciting. It really was," he says. "I spent all of my youth there and I became successful while I was living there. So [it's] a very special place. It was a project that was a trial thing that the government was doing — putting together government projects for people who weren't making a lot of money, and you paid rent there according to how much money you made a week."

"All my dreams originated from there. From being a kid growing up there and my mom taking me at a very young age to see my very first performance, Sinatra. I had made up my mind that that's what I wanted to do. I had no idea how I would get to that place."

The singer — who was inducted into the Rock and Roll Hall of Fame in 1990 — remains utterly grateful for his life and career, which led him to experiences like becoming friends with Sinatra: "Never thought in a million years that I would make it or become friends with Frank Sinatra, but it happened and it was incredible."

He also shared something he learned from the "Strangers in the Night" singer, who died in May 1998: "You learn how to be humble, and you learn to understand that you're only a star in any sense of that word if you have an audience, and the audience is really the most important thing. Without an audience, I don't care what your name is, it wouldn't matter."

"Living in the projects was good for me for so many reasons. It gave me the incentive to want more, and although I didn't get a full education — no one was sending me to college anyway so it wouldn't have mattered — I had to do something to make something of my life. [I had] to come above everything that I grew up with."

A piece of advice from Valli? "Everybody should live the life they want to live without hurting anybody. That's the secret."

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