I would imagine that there is no perfect age to learn to throw a sinker, since young people grow at a varying pace. However, if I were the parent of a young baseball player, I would talk with his pediatrician about the matter. His doctor will know how children grow and what stresses they can withstand during certain growth periods. Also, it would make sense that learning to throw a sinker, would be less dangerous or stressful on muscles, ligaments, tendons etc., as the child nears full growth potential. The best suggestion is to ask a certified health professional, especially one that specializes in sports medicine.