Most health organizations including Mayo Clinic now believe children age 7-18 can gain significant benefits from SUPERVISED strength training1:
...Done properly, strength training [for kids] can:
- Increase your child's muscle strength and endurance
- Help protect your child's muscles and joints from sports-related injuries
- Improve your child's performance in nearly any sport, from dancing and figure skating to football and soccer
- Develop proper techniques that your child can continue to use as he or she grows older
... strength training isn't only for athletes. Even if your child isn't interested in sports, strength training can:
- Strengthen your child's bones
- Help promote healthy blood pressure and cholesterol levels
- Help your child maintain a healthy weight
- Improve your child's confidence and self-esteem
The New York times reports that beginning supervised strength training between 7 and 12 years of age can significantly improve muscular function and efficiency.2
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1 www.mayoclinic.org/healthy-lifestyle/tween-and-teen-health/in-depth/strength-training/art-20047758
2 well.blogs.nytimes.com/2010/11/24/phys-ed-the-benefits-of-weight-training-for-kids/