In humans your Endocrine system is not fully functional until an average age of 25yrs, although the main development is up to around 21yrs it still fluctuates a little bit up to its fully functional age. There is a risk of permanently damaging your HPTA if you take AAS to young and you could end up with symptoms of andropause and HRT for life. Symptoms could be Limp dick, low libido, depression, low energy, low endurance, erection problems and many more but.......are these the types of symptoms you want to have in your 20's?. Believe me its hard to cope with these in your 40's yet alone in your prime of your life.