This depends on two factors. First, are you throwing and catching the medicine ball (dynamic exercises), or are you using it more statically? For throwing and catching drills you will want medicine balls that are lighter in weight. Here is a general guideline: Up to 12 years old, have children use their own body weight or the lightest 2-kilogram medicine balls. A girl through high school will generally stay with a ball no heavier than 3 kilograms. Boys may go to 4 kilograms. Past high school depends on the size and strength of each athlete.