As a pediatrician who sees many pre-teens and teenagers, I am frequently asked "My child wants to take ______ (sports supplement). Is it safe?" I explain that the current supplements available have not proven to be safe and effective. It is possible to have sports performance worsen because of supplements. In addition, the short and long term safety of these are in doubt, with side effects including kidney damage, growth issues, stomach issues, heart issues and high blood pressure all under investigation.
Here is a link about safely improving sports performance. Here is a link to a different article about the sale of these products to under-age teens. It is good to remember the store is trying to make a sale, not look after your safety. Be very skeptical about claims these sports supplements.
The safest way to gain strength and muscle is to add lean protein in your diet. If you Google search this, millions of articles come up. Some of these, sadly, will lead you to purchase their supplements (which I hope our patients will avoid)! Here is an article with good information about good sources of protein.