The human life span may have finally plateaued. Here's what you can do to optimize your chances for a long life — and what researchers are working on next.

• There's a lot that a healthy lifestyle can do for your longevity.
• But in old age, a big part of the equation comes down to genetics.
• A new study suggests only a small percentage of adults will live to 100, barring a big breakthrough.

We are, collectively speaking, living longer than ever before.

Life expectancy in the US shot up at a rate of about three years per decade in the last century: from roughly age 47 in 1900 to age 77 in 2000. This was largely due to major medical breakthroughs and public health moves that boosted longevity, including handwashing, proper sanitation, nutrition, and exercise.

An analysis published Monday in Nature Aging suggests that our longevity-boosting heyday is coming to a close.

As more people in wealthy countries make it to old age, human life expectancy gains are nearing a plateau. There are no demographic hints to suggest that most of us alive now will make it into the triple digits, no matter how hard we try, how rich we may become, or what any biological age test might tell us. It appears there are some strict limits to what medicine and biology can do for the human lifespan, at least right now.

"If somebody tells you that they know how to get you to 100, don't listen to them because they don't know what they're talking about," lead study author S. Jay Olshansky, a biostatistician who studies the upper limits of human longevity at the University of Illinois, Chicago, told Business Insider.

His assertion is that, barring any big advances, only 15% of women and 5% of men may live to 100 in this century. This best-case-scenario estimate is bolstered by 30 years of demographic study on some of the longest-lived, richest countries in the world, from 1990 to 2019.

But, he says, this isn't cause for despair. It's a reality check.

"We've got a lot of people making it out to 85 or 90," he said, but "100 isn't going to happen for most people."

The study has ramifications for aging science too, because stagnation in the life expectancy curves means longevity scientists will need to develop new, creative tactics to deliver major life expectancy gains for humanity in the decades to come.

There is still a lot we can do to live long lives

Olshansky's new study includes data from the US, plus eight of the longest-living countries in the world (Australia, France, Italy, Japan, South Korea, Spain, Sweden, Switzerland ) and the one place which, until very recently, consistently boasted the highest life expectancies on Earth (Hong Kong). The study tracks trends in their death rates and life expectancy from 1990 to 2019, before the COVID-19 pandemic started.

He says his projection doesn't mean that personal health choices are unimportant.

"The first take-home message is: avoid the things that shorten life," he said. "One of the reasons why people often don't live as long as they can is because they adopt unhealthy lifestyles."

Some of the best anti-aging interventions we have today include healthy, vegetable and whole grain-heavy diets combined with ample exercise.

There is serious science suggesting that lifting weights can help you maintain mobility, boost bone health, and reduce pain. Cardio like running, walking, swimming, and biking has benefits that go beyond just keeping your heart healthy or your blood pressure down. It can help protect the brain from cognitive decline and lowers your risk of early death. Scientists often say that exercise is the best anti-aging drug we've found so far.

A lot of what determines who makes it past the century mark comes down to genetic luck.

"When somebody says 'What's the best advice you can give to somebody on how to live to a hundred?' My answer is: choose long-lived parents," Olskansky said. "There's a strong genetic component of how long we can live, and we can't really control that. So control what you can control."

New ways to slow down aging

Aging researchers generally agree that new tactics beyond traditional medicines, vaccines, and supplements will be needed to improve human health and longevity at very old ages.

"We've created a toolbox in medicine to go after one disease at a time," Olshansky said. In order to advance further, aging science "needs to go after the aging process itself," he said.

Future advances could come from innovations in genetics, cellular reprogramming, and other areas of biology.

"Big breakthroughs in a technology only happens as a result of a lot of prior work that doesn't have any actual dramatic consequences, until it does," aging researcher Aubrey de Grey told Business Insider. De Grey says he is focused on "damage repair" interventions against aging and is studying regenerative medicine in mice that could lead to breakthroughs for human lifespan extension. He's currently testing different combinations of stem cell treatments, drugs (including one that's a potential "zombie cell" killer), and a gene therapy.

"The goal is to throw together enough of these to be able to essentially cover all the bases," he said.

Physicist and aging researcher Peter Fedichev is another aging scientist who envisions paradigm shifts that could help slow down the aging process itself. His AI-based drug company, Gero, hasn't developed any treatments tested in people yet, but it's partnering with drug companies including Pfizer, working on research to develop new targets for fibrotic disease.

"If we keep doing what we have been doing, the solution does not appear to be there," Fedichev told BI. "We need to decouple aging and diseases in our minds, and we should put effort into designing interventions against aging, to prevent that decline."