Every fitness enthusiast certainly wonders at what age their hard work will start leveling off and then begin declining due to the natural aging process.
It’s like a train coming at you, and there’s no way to stop it.
A 47-year Swedish study found that aerobic fitness, muscular endurance and other physical abilities generally peak between 26 and 36 before gradually decreasing.
The good news? You can still build strength and improve fitness as you get older.
If you’re 60, for instance, and have always been sedentary, you can dramatically improve your fitness — aerobic and strength — with a structured exercise program.
- You’re not doomed to stay flabby and listless just because you’re 60 or even 70.
- A deconditioned older body WILL respond to a training stimulus — every single time.
But What Happens to Fitness After 35?
Researchers at Karolinska Institutet followed 427 Swedish men and women born in 1958, testing their physical abilities repeatedly from 16 through 63.
Most participants reached peak aerobic fitness and muscular endurance between ages 26 and 36. Some types of explosive muscle power peaked earlier.
Afterward, performance gradually declined, with the drop becoming more pronounced with age.
By 63, average performance had fallen 30% to 48% from peak levels for both men and women.
Can Exercise Slow the Decline?
Although exercise didn’t completely prevent age related changes, it helped people maintain their physical abilities.
Participants who became more active during adulthood improved their physical capacity by 5% to 10%.
Those who exercised as teenagers also tended to retain better fitness and muscle power later in life.
The study, published in the Journal of Cachexia, Sarcopenia and Muscle in 2025, reinforces an important point: Starting exercise later in life can still make a difference.
Your 30s aren’t a deadline for building muscle or improving endurance.
Your 30s aren’t a deadline for building muscle or improving endurance.
Why Does Physical Fitness Decline?
Researchers are still investigating the reasons behind these changes.
Aging can affect muscle cells, the nervous system and the body’s ability to produce energy.
Muscles may gradually lose mass and become less responsive to signals involved in growth and repair.
Over time, these changes can contribute to sarcopenia, the age-related loss of muscle mass and strength.
Some of the underlying processes may begin decades before noticeable weakness develops.
Fitness and Artery Health
A 2026 study in Scientific Reports found that people with better aerobic fitness at ages 34 and 52 tended to have less arterial stiffness at 63.
Stiffer arteries are less flexible and are associated with increased cardiovascular risk.
Other research supports staying active well beyond middle age.
Some physical decline is natural, but getting older doesn’t mean becoming weak or inactive.
Studies are clear that an exercise regimen will improve strength, endurance and overall fitness at virtually any age.
