21/02/2026
427 people. Same fitness at 16. Measured again at 23, 30, 37, 44, 51, and 63.
By the end, the most and least active weren't even on the same chart anymore.
Everyone's aerobic capacity peaks somewhere in their late 20s to mid-30s. After that, everyone declines. Slowly at first, then sharply after 50. That part is inevitable.
What isn't inevitable is where you end up. The spread between the most and least active by 63 was enormous, and it wasn't explained by where they started.
Here's the part that matters if you feel like you missed the window: people who were inactive early but became active later didn't just slow their decline. They reached their personal peak fitness later than the always-active group. If you weren't training at 25, your body can still be improving at 45.
The caveat matters though. Your late-arriving peak will almost certainly be lower than it would have been if you'd built that base earlier. The ceiling is shaped by what you did in your 20s and 30s. But a lower peak you actually reach beats a theoretical one you never did.
The fan in this chart isn't genetics. It's decades of accumulated decisions about movement, and those decisions compound in both directions.
Westerstahl M, et al. J Cachexia Sarcopenia Muscle. 2025;16(6):e70134.