(HealthDay News) -- Your preteen has told you he wants to start pumping iron, but you think he's too young and could hurt himself.

Well, if your son is 12 years old or older, weight training is generally safe, provided the program is supervised and properly designed, according to St. Louis Children's Hospital. Young girls usually can lift weights from age 10.

But tell your child not to expect too much buildup of muscle mass, since this requires certain hormones that pre-adolescent kids lack. Weight training at this age will increase muscle fibers to develop strength, but not muscle mass.

It's also a good idea for your child to have a medical checkup before starting a weightlifting program.