Not only is there life after 90, but it can be a vibrant and productive one as well.

So say show business legends Carl Reiner, 95, Mel Brooks, 90, Dick Van Dyke, 91, and Norman Lear, 94, who appear in a charming new documentary, If You're Not In The Obit, Eat Breakfast - a look at ageing well that also features Betty White, Kirk Douglas and other remarkable nonagenarians.

Earlier this week, the lively foursome shared their tips on ageing well with The Straits Times and other guests at a Los Angeles screening of the film.

Their mantras are to keep working, keep moving and laugh as much as they can, they say. And "eat bran", quips Brooks, who paces the stage like a man half his age as he keeps the room entertained with funny stories.

He and the others say society must let go of its fear of ageing. This is a theme in their inspiring movie, which begins with a live performance of Tony Bennett crooning the jazz standard The Best Is Yet To Come and ends with Van Dyke singing Young At Heart.

The documentary's title comes from a funny story Reiner likes to tell about his morning routine. "I go down the list of obituaries every morning and go, 'Got you beat. Got you beat."

The prolific comedian, writer and director behind classic films such as The Jerk (1979) still sits at his computer and writes every day, authoring several memoirs and also doing the occasional guest-acting stint on sitcoms such as Two And A Half Men

"Dick says, 'Keep moving.' I say, 'Keep typing.' I get up in the morning because I've got two books coming out and one just came out, called Too Busy to Die."

The nine-time Emmy winner - whose son is When Harry Met Sally (1989) director Rob Reiner, 70 - is also young at heart when it comes to social media.

One of the oldest celebrities active on Twitter, he makes a point of using the platform to speak out against United States President Donald Trump. "I cannot go to bed unless I do an anti-Trump tweet."

The documentary begins with Reiner observing that many of his friends aged 90 and older appear to be thriving and some are as creative and productive as ever.

He sets out to discover why (is it genes, luck, modern medicine or some other factor?) and also to correct what he says are common misconceptions, mostly negative, about being that age.

One of the first to be interviewed in the film is his old pal Van Dyke, who starred in Reiner's Emmy Award-winning sitcom The Dick Van Dyke Show (1961 to 1965).

Van Dyke still sings, dances and performs daily with as much vigour as before, now often performing with his wife Arlene, his former make-up artist who is 46 years his junior.

"I feel as good as I ever did," declares the five-time Emmy winner, who has four children from his first marriage.

Van Dyke is still a working actor - he just finished filming a remake of another one of his movie musicals, Mary Poppins (1964), which co-stars Emily Blunt as Mary and is due out next year.

"I got to do a song-and-dance number. I did good," says Van Dyke.

He also wrote a book on ageing, a 2015 tome titled Keep Moving: And Other Tips And Truths About Aging, which he jokes was originally called What To Do While Circling The Drain.

Also interviewed in the documentary are Oscar-winner Brooks, the comedic genius behind hit films such as The Producers (1968) and Blazing Saddles (1974); The Golden Girls (1985 to 1992) star White, 95, who still acts and writes books; and Spartacus (1960) actor Douglas, 100, who performed a one-man show in 2009 despite having a speech impediment from a stroke.

Non-celebrities featured in the show include 102-year-old athlete Ida Keeling, who began running competitively in her 60s; and World War II paratrooper Jim Martin, 96, who is seen jumping out of a plane to commemorate the 70th anniversary of D-Day three years ago.

The film, which airs on HBO in the US next month, also consults longevity expert Dan Buettner, who has studied long-living populations in places such as Okinawa, Japan, and Sardinia, Italy.

His research seems to confirm what Reiner and the others say: The key to staying vital in one's 90s is to remain useful and productive so there is a reason to get up in the morning.

•There is no local release date yet.