Most of us have a vague sense that fitness fades with age. Few of us put a number on when it starts. A new Swedish study puts a sharper edge on that story.
Physical performance peaks in the mid-30s and then starts to slip, even in people who stay regularly active. That is the central finding of a 47-year study of 427 Swedes published in the Journal of Cachexia, Sarcopenia and Muscle in December 2025.
Researchers at Karolinska Institutet followed the same men and women from age 16 to 63, repeatedly measuring their aerobic capacity, muscular endurance and explosive power.
Aerobic fitness and muscular endurance both peaked between ages 26 and 36. Explosive muscle power, captured by a jump test, peaked earlier still. By age 63, the group had lost roughly 30 to 48 percent of their peak performance across those measures.
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What exercise can and cannot do for the curve
The study is blunt about what regular training does and does not fix. Decline still comes. But participants who became physically active in adulthood improved their performance by about 5 to 10 percent compared with inactive peers.
The researchers put the message plainly. It is never too late to start, they say, and physical activity can slow the decline even if it cannot stop it entirely.
Why the mid-30s drop matters even if you still feel fine
The slide that begins in your mid-30s is quiet at first. It is initially modest and becomes progressively faster with age.
The losses accumulate. By 63, the gap from peak is wide enough to show up in daily tasks like climbing stairs, carrying groceries or getting up off the floor.
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Starting in your 60s still delivers measurable gains
A separate line of research confirms that exercise still works late in life. A Danish trial published in BMJ Open Sport and Exercise Medicine followed 451 adults around retirement age for four years.
The participants were randomized to one year of heavy resistance training, one year of moderate exercise, or no exercise program. Only the heavy resistance group still had their leg strength preserved four years later.
That dovetails with the Karolinska team's practical read of their own numbers. The curve bends down from the mid-30s onward, but the people who stay active sit higher on it at every age.
Westerståhl's team plans to retest the Swedish cohort at age 68, linking changes in physical capacity to lifestyle, health and biological mechanisms.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.