The endocrine system is comprised of over 100 hormones produced by glands in the body. They begin to mature at 18 and is not fully mature until age 25. Yes, the male reproduction system matures early at 18 but other systems do not mature until age 25. The reason 25 is a popular age to start AAS is because almost all of the glands of the endocrine system are mature not just the sexual reproduction.
Your second correction is technically correct. However, it's a easier to say that the endocrine system control the test because it require less explanation. The main point that I am emphasizing is that the network of glands in the endocrine system whether they be in the testes or hypothalamus is the basis from which level of testosterone is controlled.
You draw a general conclusion on the HGH based on false assumption of endocrine system. The secretion of HGH is highest at the age of 18-20 and tapers off over time. At age 30 is when the decline of HGH secretion can't keep up with the cellular deterioration. If you're giving advice to 18you to take AAS because their endocrine system has matured you should be careful. Also, having people younger then 30 start on HGH is not healthy. In your enthusiasm to express your knowledge you're giving bad advice to young people.
http://today.msnbc.msn.com/id/10766061/
http://www.antiaginggroup.com/
Liu H, Bravata DM, Olkin I, Nayak S, Roberts B, Garber AM, Hoffman AR (January 2007). "Systematic review: the safety and efficacy of growth hormone in the healthy elderly". Ann. Intern. Med. 146 (2): 104–15