Stop guessing: A massive 50-year study finally reveals when our bodies really start to age

a senior woman holding dumbbells — (Image credit: Shutterstock)

A 47-year-long Swedish study set out to provide new insights into when our bodies start to age by looking at how fitness, strength, and muscle endurance change during adulthood. The results? Our bodies begin to deteriorate as early as age 35, but there’s good news — you can slow your performance loss down by moving more.

The study, which was conducted at the Karolinska Institutet and published as part of the Swedish Physical Activity and Fitness study (SPAF), followed several hundred randomly selected men and women, all of whom were born in 1958. Researchers assessed the participants’ maximal aerobic capacity, muscular endurance, muscle power, and physical performance using various athletic tests.

The scientists found that the general population showed similar changes in physical capacity to those previously demonstrated in elite athletes — peak physical performance is reached before age 35.

a senior woman doing exercise — (Image credit: Shutterstock)

Maria Westerståhl, lecturer at the Department of Laboratory Medicine and lead author of the study, said, “It is never too late to start moving. Our study shows that physical activity can slow the decline in performance, even if it cannot completely stop it. Now we will look for the mechanisms behind why everyone reaches their peak performance at age 35 and why physical activity can slow performance loss but not completely halt it.”

The study continues, and this year the participants will turn 68 and be examined again. The researchers hope to link the changes in physical capacity to lifestyle, health, and biological mechanisms.

What are you waiting for? If this isn’t motivation to keep moving in 2026, we don’t know what is!

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Jane McGuire is Tom's Guide's Fitness editor, which means she looks after everything fitness related - from running gear to yoga mats. An avid runner, Jane has tested and reviewed fitness products for the past five years, so knows what to look for when finding a good running watch or a pair of shorts with pockets big enough for your smartphone. When she's not pounding the pavements, you'll find Jane striding round the Surrey Hills, taking far too many photos of her puppy.

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