Not age. A 25-year-old has no inherent advantage over a 23-year-old. American runners tend to peak later because they have more years of aerobic training, but that advantage is negated if you spend 2 years on a mission with minimal exercise. Many talented runners at BYU lose interest in running when they go on a mission and never get back in shape. But the ones who stick with it succeed due to their dedicated training and avoidance of vices such as alcohol.