When you are under 21, your endocrine system is still developing. This should be obvious as you are still going through the end of puberty, getting acne, etc. During this time period, supplementing with exogenous hormones is EXTREMELY DANGEROUS. There is potential for very serious and long term side effects to occur. I hear all the time some of these people saying that "I've stopped growing, so it's okay". No - it's not okay. There are many other potential side effects besides growth plates. Here is a list of a few.