Krato Sports recommendations are based on repetitions rather than heavy use.

10-25oz: Ages 8-10 depending on size and strength of child.
15-30oz: Ages 10-13 depending on size and strength of child.
15-30oz: Ages 13-16  depending on size and strength of child.

40oz: Recommended for High School, College, Pro etc. This weight is very heavy and not recommended for most children's swing development.

In general, kids and teens should tone their muscles using light weights and a high number of repetitions rather than lifting a heavy loads.