You might not qualify for any senior-citizen discounts yet. But aging starts sooner than you might expect.

Age-related hearing starts going downhill at 25, though it isn’t noticeable until decades later. We start losing bone mass as early as our 30s. And a recent study by Duke University researchers found that some types of physical decline—particularly lower-body strength and balance—often begin in the 50s.

“Every function of the human body declines 5% every 10 years,” says Michael Roizen, chairman of the Wellness Institute at the Cleveland Clinic. “That’s brain function, heart function, liver function. The difference is when you sense it and when it hits the critical level where it decreases functioning for you.”

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The physical performance study, published in July in the Journals of Gerontology: Medical Sciences, looked at 775 people from their 30s to over 90. The participants took five functional tests measuring strength, balance and endurance, including standing on one leg for a minute and rising from a chair repeatedly for 30 seconds.

In general, younger people performed better than older people and men better than women, as expected, says Miriam Morey, a professor of medicine at Duke University School of Medicine and senior author of the study. But researchers were surprised to find a marked decline in performance on the balance and chair test starting when participants were in their 50s.

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