Michael Phelps is physically stronger and hardier than most human beings because his genetics are slightly different (his red blood cells are smaller but more numerous, for example, so his blood is more efficient at carrying in oxygen and nutrients and carrying away waste.) Mozart was an absolute musical
genius who got started at age 5. Gauss was likewise a mathematical genius who (at least apocryphally) was proving meaningful results
in grade school. Meanwhile, some folks have dyscalculia or dyslexia or dyspraxia or a host of things that impede learning, or have physical weakness outside the bounds of normal, or suffer from a congenitally weak constitution, or absolutely chronic foot-in-mouth disease (

), etc., etc. Factors that can easily push someone well outside the "norm" for their physiology.