Weight training can be an invaluable part of overall fitness and compliment the many other sports and activities that your teen may already participate in or want to do.
Does your son wish to dead lift or do light weight repetitions?
Doing light weight repetitions, if supervised properly, is great for teens and even for younger kids. Weight training can be a great way to build bone density and balance, but it is important to have proper form and technique and match the light weight properly for the child's ability.
Dead lifting is another situation altogether. This requires a certain level of maturity, both physically and emotionally, to be productive and safe. If your teen wishes to dead lift, it is recommended that he or she be past his or her adolescent growth spurt (AGS). See Effects of Puberty on Sports Performance: What Parents Need to Know for more information.
What is your son's present stage of puberty?
Boys can gain about 40 pounds over the course of puberty, most of it muscle. Once the AGS is complete, the muscles have the ability to enlarge. Many parents and teens don't realize that it is not possible to enlarge or sculpt muscles during earlier stages of development. There is also a greater risk for injury prior to the AGS.
Who will be supervising?
It is always of the utmost importance that dead weight lifting be supervised with someone qualified to do so properly. Many serious and permanent injuries have resulted from improper use and lack of supervision of this very dangerous equipment.
Like most things though, when done well and properly there can be good value. Be careful and be safe.
Additional Information from HealthyChildren.org: