95-year-old comedy legend Carl Reiner wonders why some of his friends died years ago, while others are still thriving. "Was it luck, genes, modern medicine or are we doing something right?"

"Every morning before having breakfast, I pick up my newspaper and get the obituary section," joked Reiner, in his new HBO documentary. "If I'm not listed, I'll have my breakfast."

Reiner tracks down such celebrated nonagenarians and centenarians as Mel Brooks, 90, Dick Van Dyke, 91, Norman Lear, 94, and Betty White, 95, who are proof that the later years can truly be the happiest and most rewarding. The film's opening title song, "The Best Is Yet to Come," is performed by Tony Bennett, 90, and an original song "Just Getting Started," was commissioned for the show.

"At 30, I exercised to look good. In my 50s, I exercised to stay fit. And in my 90s, it's just gonna be out of defiance," says Van Dyke.

You'll see comic book writer and publisher Stan Lee, the former president of Marvel, who's still writing, producing and making cameo appearances in hit films at 94 and fashion icon Iris Apfel, at 95. Actor Kirk Douglas, 100, chats with Reiner about the stroke that left him unable to speak and the subsequent one-man show he undertook, with his wife's encouragement, to prove he could still function.

Also interviewed are Harriette Thompson, 93, the oldest woman to run and finish a marathon, Ida Keeling, 101, a track and field athlete, who works out for an hour every day, 98-year-old yoga teacher, Tao Porchon-Lynch, who marched with Gandhi and recently took up tango, and Jim "Pee Wee" Martin, 95, who fought in D-Day and still parachutes.

Younger celebrities, too, are featured in the documentary, including comedian Jerry Seinfeld, who's already reserved the stage at Caesars Palace for his 100th birthday show; and Van Dyke's much-younger wife, Arlene, who's got all she can handle keeping up with her exuberant husband on the dance floor.

(WATCH the trailer below, or full film on HBO Go or HBO Now)

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