A myth that has been around for years and is believed by many is that weight lifting at a young age is harmful to the body and may also stunt growth. Many scientists, doctors, nutritionists, and other specialists in this field have done intense research to label this myth as ‘invalid’.
The truth is that weight lifting remains as one of the safest sports when done CORRECTLY. Olympic weight lifters like Arnold Schwarzenegger started at a very young age to make it to the top. With proper training and guidance, the technique is perfected and assures safety while working out. The myth itself is around simply because there are huge possibilities and many cases in which injury from lifting weights has caused growth stunt in the human bodies. Humans have growth plates at the end of many bones that maintain length and regulate the shape of the bones as they grow. The growth plate is the last part of the bones to develop and mature, therefore, making them very prone to breakage/fracture. In the cases where children perform improper techniques while lifting, one mistake can injure their bones for good.
In an article written in the New York Times in 2010, Gretchen Reynolds, discusses two separate notions regarding this topic. At first, Reynolds quotes a professor in science at the College of New Jersey who states “that children and adolescents should not practice weight training” and that it will “result in short stature, epiphyseal plate” — or growth plate — “damage, lack of strength increases due to a lack of testosterone and a variety of safety issues.”
Reynolds goes on to state that researchers in Germany analyzed the information and the charts of 60 years worth of studies of children and weightlifting. The ages of the children raised from 6-18. Through the study, the researchers were able to confirm that weight lifting at a young age is indeed beneficial to the human body. While some may believe that weight training at such a young age causes nothing but injury, it is scientifically proven that lifting weight increases muscular as well as bone strength making the child LESS susceptible to bone/tissue injuries. 
http://www.livestrong.com/article/430166-can-lifting-weights-when-young-stunt-your-growth/
During my research, I came across a lot of doctors who were quoted saying that it’s not exactly the amount of weight the child is lifting. It’s even beneficial if they simply use their own body weight (pushups, pull-ups, etc.) In other words, what you’re saying is absolutely correct. As you age and your body develops, the amount of weight you use and even the amount of reps should increase. Starting young develops healthy habits though that tend to lead to a better, even more structured lifestyle.
What would you say to the notion of joint damage such as shoulders, knees and hips from too many repetitions starting at an early age? For example shoulder pressing and bench pressing at an early age, no matter how good the technique, is bound to end with the need for shoulder joint replacements because of wear and tear. Same goes for the knees and back with squatting.
I feel as though lifting at a young age is definitely beneficial yet in certain amounts. A child should not be lifting the same way someone older should lift. For example people who are over about 16 should be lifting heavier weights so that they build mass yet a child under this age should go for lower weights higher reps to promote healthy muscles and better definition . this will allow the muscles to grow more defined then once the child comes of age he or she should begin to build mass.