I know, I know . . . we've always been told "NO!"  "Wait until they're done growing in height."  "It can stunt growth."   <---------- MYTHS!!!!!

   However, the evidence shows these fears are largely unfounded.
LINKS PROVIDED for each siting:     http://www.reuters.com/article/2010/10/26/us-strength-training-idUSTRE69P3M120101026

The most recent meta-analysis, published in the journal Pediatrics,
found that children who engaged in strength training using either free weights or resistance-training machines 1-5 times a week, for about 40 minutes per session, improved their strength by 20%-40%!!!!

Teens experienced even higher levels of improvement, compared to kids under the age of 10.
Those who trained a few times a week also saw more improvement than those who limited their strength training to once a week.
The most effective exercises included isotonic contraction-type exercises, such as:
•Bicep curls
•Squats
•Bench presses

These findings line up with the most recent, updated exercise recommendations from the National Strength and Conditioning Association (NSCA),
which now recommends children, aged 6 and older, to incorporate strength training 2-3 times a week.

As of last year, the position of the NSCA is that a properly designed, supervised and age appropriate resistance training program for youths can:

•enhance muscular strength and power
•improve cardiovascular risk profile  
•improve motor skill performance and may contribute to enhanced sports performance  
•increase a young athlete's resistance to sports-related injuries
•help improve the psychosocial well-being
•help promote and develop exercise habits during childhood and adolescence
Please note that it's ESSENTIAL to make sure your child is under professional supervision when using free weights or resistance-training machines, to make sure each exercise is performed safely.

Incorrectly performed exercises can cause far more harm than good, and that goes for adults as well.

The NSCA guidelines include the disclaimer that "if qualified supervision, age-appropriate exercise equipment, and a safe training environment are not available, youth should not perform resistance exercise due to the increased risk of injury."