Exercise stimulates growth plates, by putting pressure on bone cells, forcing them to specialize and create new cells. Too much pressure, however, damages the growth plates, which are soft and fragile...The rule of thumb is that lifting one's own body weight (in push-ups,pull-ups, etc) is fine at any age, and kids over fifteen can lift weights.
Saul had gained his six-foot frame at sixteen, but his muscles didn’t arrive until his early twenties. Between those lost years, he was a gangly, uncoordinated klutz. He was told that he could improve his dancing by watching himself in the mirror. He tried. What he saw was so repulsive that he resolved never to inflict himself on a dance partner. These days, Saul hid those memories behind weight lifting and jogging. His new athletic physique hid his aimless decade as an outsider, an odd and lonely kid--as he remembered it.
Thinking is like exercise, it requires consistency and rigor. Like barbells in a weightlifting room, the classics force us to either put them down or exert our minds. They require us to think.