there's no magic age bro. but let's say this, when you're around 25 pretty much everything in your body is done growing and all hormonal changes s/b set. now all this might also occur at age 18, but i don't think i'd like to take a chance on messing something up. now imo, and i'm not a MD but i would think that around age 21 or so you s/b done with all i have mentioned but it is a personal decision on what age, if any you try ph/ps/aas.
i know that steroids have been blown out of proportion by the media for whatever reason but there are real dangers out there. user beware. have your research done and then do more before even buying anything