What makes movie stars ageless? (Besides surgery.)

FORTUNE -- The anti-aging quest got some attention this week when actor/producer Ashton Kutcher was pictured in a British tabloid reading my book, The Youth Pill, on a beach next to his wife, actress Demi Moore. According to one account, Moore had given the book to Kutcher -- the smirking implication was that he needed all the help he could get as the aging consort of the seemingly immortal Demi. The uncannily svelte, 48-year-old mother of three was photographed in a skimpy black bikini, leaving no doubt about her all-around youthfulness.

After being duly impressed, I found myself wondering: Why do so many movie stars seem to age well? And at a time when so many aging workers are trying to find ways to stick it out just a little bit longer in the workforce, is there something we can learn from Hollywood to help our longevity?

Of course, many stars have aged badly and faded out all too soon. But Hollywood's beautiful people have come a long way since the days when hard-drinking, chain-smoking bombshells and tough guys flamed across marquees bearing a closer resemblance to meteors than stars. In fact, Moore is just one of many ageless wonders in Tinseltown -- take a look here and here.

But how real is the ageless-star phenomenon? After all, Hollywood celebrities are probably the most mirror-obsessed, self-pampered people on earth, and their personal assets are rigorously tended by certified-genius plastic surgeons, throngs of personal trainers, and kitchen staffs headed by famed French chefs consulting with Harvard-trained nutritionists. So the phenomenon may be no more than a skin-deep illusion.

Still, an unusually large proportion of stars seem durable both inside and out. Betty White seems in her prime at 89. Tony Curtis continued acting into his 80s, last appearing in a movie that came out when he was 83 (he died at 85). Jessica Tandy won an Oscar at age 80. Katherine Hepburn, who died at 96, continued appearing in films through her mid-80s. George Burns and Bob Hope made it to 100, and both continued to make stage appearances well into their 90s. In its lists of centenarians by profession, Wikipedia names more than 50 actors, compared with 18 medical professionals, 9 philosophers and theologians, and 9 explorers.

Statistics don't lie

As a student of aging science, I'm much intrigued by Burns and Hope. Male centenarians are extremely rare. The Census Bureau estimates that in 2010 there were only 70,490 Americans over 100, and the great majority were female. A back-of-the-envelope calculation suggests male centenarians make up something like 0.005% of the U.S. population. It doesn't take a statistician to see that the odds of two of them turning up in a randomly picked group of a few hundred American men would be very low. (I mention a few hundred because that's my rough guesstimate of the size of the very select group Burns and Hope belonged to -- male Hollywood superstars of the 20th century. True, there are over 2,400 stars embedded in the Hollywood Walk of Fame, but many of them represent non-actors.)

If there were, say, 400 male superstars during the 20th century, Burns and Hope would represent 0.5% of the group -- that's 100 times the rough percentage of male centenarians in the general population. You might dismiss this statistical oddity as a meaningless fluke. Then again, it might suggest there really is something special about actors and aging.

So what is it? Their riveting good looks, at least as young adults, might mean that those with star power tend to be blessed with a whole array of high-quality physical attributes, including ones that make for slow aging. But even if this idea has some validity, I doubt that it's the whole story.

For one thing, most of the gene variants known to retard aging in mammals -- a half dozen have been discovered in mice over the past 15 years -- also cause dwarfing, which is quite rare among the silver screen's big names. In fact, scientists have just reported that similar gene variants appear to slow aging in a group of Ecuadorian villagers who are generally less than three and a half feet tall. (I do know of one extraordinarily long-lived Hollywood actor, however, who was a dwarf: Meinhardt Raabe, who played the Munchkin coroner in The Wizard of Oz, made it to 94. He died last year, and appeared on an NPR talk show only a few months before his death.)

Never grow old with laughter

All the high-class pampering may be a factor, but actors are also notoriously self-indulgent when it comes to life-shortening vices, so I doubt that it explains the phenom. Besides, scientists who study centenarians have been unable to identify any lifestyle secrets that could explain their extraordinary longevity. Many centenarians have favored heart-attack-on-a-plate foods all their lives, and some have even been smokers -- Jeanne Calment, who holds the record for longevity, 122, reportedly had a two-cigarette-a-day habit for over 90 years.

But many centenarians do share a trait that I suspect bears on the ageless-star question: They tend to be remarkably resilient, capable of shrugging off setbacks and losses that would leave many of us psychically scarred for life. I suspect many big stars share this quality -- it's likely that only highly resilient people make it through the demoralizing gantlet that actors face on their way to the top: the struggles with stage fright (which apparently never ends for many), the endless slogging through bit parts in duds, the withering reviews by snide critics, and worst of all, flops before live audiences.

Comedians may face the toughest gantlets of all -- it's hard to make people laugh night after night -- and the ones who succeed over long careers are probably off-the-charts resilient. Consider how George Burns, though staggered by Gracie Allen's death in 1964, bounced back to become a bigger star than ever.

Thus, my guess is that the ageless-star phenomenon may have to do with the combination of genetic and early-life influences that shape brain chemistry, not external physique. If so, Ashton's experience as a comedian suggests that he needn't worry too much about keeping up with Demi when it comes to graceful aging.

David Stipp is a freelance science writer, formerly with the Wall Street Journal and Fortune, who has covered gerontology since the late 1990s and writes a blog on aging science.