It is never too late to start moving
For nearly fifty years, researchers in Sweden have followed the same individuals from adolescence into their sixties, tracing the arc of physical capacity across a single lifetime. What they have found is that the body begins its quiet retreat around age 35 — a threshold not of failure, but of turning. Yet even in that long descent, the body remains a willing partner: those who begin exercising later in life still gain measurably in strength and endurance, a reminder that the relationship between effort and response does not expire with youth.
- A landmark 47-year longitudinal study from Karolinska Institutet has confirmed what many suspect but few can prove: physical decline begins reliably around age 35, not in old age.
- Unlike snapshot studies that compare different people at different ages, this research tracked the same individuals across five decades — making its portrait of decline unusually precise and difficult to dismiss.
- The erosion of fitness, muscular strength, and endurance is consistent and accelerating, meaning a person at 50 is measurably weaker than at 40, who was measurably weaker than at 30, regardless of their athletic history.
- Crucially, late-starters who took up exercise in middle age or beyond still improved physical performance by 5 to 10 percent — not a reversal of aging, but proof the body never stops responding to demand.
- Researchers plan to test the same cohort again at age 68, pressing deeper into the biological question of why 35 marks a peak and why activity can slow but not stop the inevitable slide.
For nearly half a century, researchers at Karolinska Institutet have been watching the same bodies age. Beginning in the 1970s with several hundred men and women recruited at age 16, the Swedish Physical Activity and Fitness study checked in on participants repeatedly — measuring fitness, strength, and endurance all the way into their sixties. The findings, published recently in the Journal of Cachexia, Sarcopenia and Muscle, are both sobering and quietly hopeful.
What makes this study unusual is its continuity. Most aging research compares different groups of people at different ages — a method that can only approximate individual change. By following the same people across five decades, the researchers built a true portrait of physical decline: the same person at 25, at 35, at 45, at 55. That continuity reveals the actual slope, not an estimate of it.
The slope is real. Around age 35, fitness begins to slip. Strength and endurance follow. The decline is not sudden, but it is consistent and accelerates over time — and it happens regardless of whether someone was athletic in youth or largely sedentary.
Yet the study carries a more hopeful finding alongside the warning. People who took up exercise later in life still improved their physical performance by 5 to 10 percent. This is not a reversal of aging, but it is meaningful: evidence that the body retains the capacity to grow stronger when asked, even as it declines. Lead author Maria Westerståhl put it simply — it is never too late to start moving.
The researchers are not finished. They plan to test the same participants again at age 68, pursuing the deeper biological questions: what drives the shift at 35, and why does exercise slow decline without stopping it entirely? For now, the message holds — the body begins to fade around 35, but it never stops listening.
For nearly half a century, researchers at Karolinska Institutet in Sweden have been watching the same bodies age. They started in the 1970s, recruiting several hundred men and women at age 16 and checking in on them repeatedly—measuring their fitness, their strength, their endurance—all the way into their sixties. What they found, published recently in the Journal of Cachexia, Sarcopenia and Muscle, is both sobering and oddly hopeful: the body begins to lose its physical edge around age 35, but it never stops responding to the effort we ask of it.
This kind of study is rarer than you might think. Most research on aging compares different groups of people at different ages—a snapshot approach that can't quite capture what happens to one person over time. The Swedish Physical Activity and Fitness study, or SPAF, is different. By following the same individuals across five decades, the researchers built something closer to a true portrait of how physical capacity changes across a lifetime. They watched the same person at 25, at 35, at 45, at 55, and beyond. That continuity matters. It lets you see the actual slope of decline, not just guess at it.
The decline itself is real and measurable. Around age 35, fitness starts to slip. Muscular strength follows. Endurance does too. It's not dramatic at first—the body doesn't suddenly fail. But the downward trend is consistent, and it accelerates as the years accumulate. A person at 50 is noticeably weaker than they were at 40, which was noticeably weaker than 30. This happens across the board, regardless of whether someone trained hard in their youth or barely moved at all.
But here is where the study offers something more than just a warning. The researchers discovered that people who took up exercise later in life—who didn't wait until they were young to start moving—still managed to improve their physical performance by between 5 and 10 percent. This is not a reversal of aging. It is not a fountain of youth. It is something more modest and more real: evidence that the body, even as it declines, retains the capacity to get stronger when asked to do so.
Maria Westerståhl, the lead author and a lecturer at the Department of Laboratory Medicine, framed it plainly: "It is never too late to start moving." The study shows that physical activity can slow the decline in performance, even if it cannot completely stop it. The body has limits, but those limits are not as fixed as they might seem. A person at 50 who starts exercising will not become as strong as they were at 25. But they will become stronger than they would have been if they had sat still.
The researchers are not done. They plan to continue following these same participants as they age further, testing them again next year when they reach 68. The goal is to understand not just that decline happens, but why. What biological processes drive the shift at 35? Why does exercise slow the decline but not stop it? These are the questions that matter for anyone trying to understand what aging actually is—not a simple countdown, but a complex negotiation between the body's inherent trajectory and the choices we make about how to live in it.
For now, the message is clear enough: the body begins to fade around 35, but it never stops listening to what we ask of it. The decline is real. So is the possibility of slowing it down.
Citas Notables
Physical activity can slow the decline in performance, even if it cannot completely stop it.— Maria Westerståhl, lead author, Karolinska Institutet
La Conversación del Hearth Otra perspectiva de la historia
Why does the decline start at 35 specifically? Is there something biological that happens at that age?
The study documents that it happens, but the researchers don't yet know why. That's actually what they're investigating next. It could be hormonal, metabolic, neurological—something about how the body is wired. But they haven't pinned it down.
So if I'm 40 and I've been sedentary, and I start exercising, I'm looking at a 5 to 10 percent improvement. That sounds modest.
It is modest. But modest is the point. You're not going to reclaim your 25-year-old body. What you're doing is fighting against a decline that's already underway. That 10 percent might be the difference between independence and frailty at 70.
The study followed the same people for 47 years. That's extraordinary. How many people stayed in the study that long?
The source doesn't specify how many completed the full span, but they started with several hundred. Attrition over that many decades is inevitable. But the fact that they have longitudinal data at all—watching the same bodies change—is what makes this different from most aging research.
If exercise can't stop the decline, only slow it, what's the point of exercising at all?
Because slowing it matters. The difference between a slow decline and a steep one is the difference between being able to climb stairs at 70 and not being able to. It's the difference between living independently and needing help. The body will age either way, but how it ages is partly up to you.
What happens next with this research?
They keep testing. The participants are going to be measured again at 68. The researchers want to understand the mechanisms—the actual biological reasons why 35 is the inflection point, and why activity works the way it does. It's ongoing work.