Depends on meaning. I encourage my patients to find physical activities that they can be engaged in for a lifetime. The idea of a workout implies something different. The aap & certified trainer groups recommend no power lifting until the final stage of physical maturity.Cardio workouts (like soccer, jogging) are proper anytime as is instruction in proper weight training by a knowledgable trainer.
Whenever. All children and adolescents should be "active". Whether or not working out includes weight training, there is no specific age when children can begin an exercise regimen. As we recommend to all patients, begin slowly, but consistently, and advance your work-outs as your conditioning improves.