Your 30s

The average age of pro triathletes is 38 for a good reason. “You’re more suited to distance than speed as you age,” says Prof Cooke.

And, over time, your body learns to build and efficiently use the key muscles specific to your chosen exercise, he says. So the amount and intensity of your training can defy the clock. What you eat matters too. “You need, on average, about 120 fewer daily calories at 35 than you did at 25,” says Cooke.

But what really slams on the brakes is less training (thank your career and family) and less lean body mass (you lose roughly 4% muscle mass per decade post 30).

“Your body weight might stay roughly the same, but it’s very common for your fat content to rise in comparison to your lean tissue composition, which means your metabolism will be slower and you’ll burn fewer calories at rest.”