What is a good age to start weight lifting?

High school or later: Around high school age, kids have more maturity (better long-term planning and dedication) and more body image awareness. They also have more testosterone, so their muscles will grow stronger with exercise. Mild or moderate weights are fine, but excessively heavy weights may be bad for their bone's growth plates. After finishing height growth (at age 16-18), they can start using heavier weights.

Any age: You can start weight lifting at any age. Generally, puberty is the most common time to start. It was long thought that weight training before puberty could stunt growth. Most recent evidence disproves this old theory. Generally, while you can gain strength and muscle coordination before puberty, you will not gain much muscle size until puberty due to hormone influence.