Hi. Since I don’t recommend supplements and am not a big fan of them – how about eating well (?), my answer would be NEVER unless the young person’s medical team (physician and nutritionist) felt it necessary because of a special circumstance or condition. A “safe age” for ANYTHING is so person-specific that I would hesitate to ever recommend something for someone based on their age alone.
I am with you on this Marlan; great links by the way. 18+ (heavy on the +) assures that the individual is at least assuming the risk of supplementation in an grown adult physique. Before the body is fully grown, improper supplementation (and while growing, most excess is improper) can negatively influence pysiological development. And definitely get with a Registered Dietitian and knowledgable health care provider on this… I know I will be. Great question.