New 47-Year Study Pinpoints When Fitness Begins to Fade (In Your 30s)

A long-running Swedish SPAF study of 427 participants shows peak physical capacity in the mid-30s, with a rise in decline after age 40 and about 30–40% loss by age 63 across aerobic fitness, endurance, and power. Lifelong activity—especially starting in youth or re-engaging later—significantly boosts outcomes and slows deterioration, but the onset of decline appears biologically programmed well before later life, highlighting the value of staying active early in life.