Its time to get on juice if you have been working out and feeding properly for at least couple of years and u r close to ur natural genetic limit and you just cant gain anymore or just gain rly rly slow.... also you should be aware of all the risks and done enough research to not fuck up lol...
I dont think its about age so much..
For example (most users will insult me for this lol):
If a guy starts working out at 13 and lets say his parents are into bodybuilding and give him a great diet and training and he trains rly hard non stop with proper nutrition n everything until hes 18... i think he would be rly close if not at his natural genetic limit and if hes done his research i dont see why not take at 18... yes it can stunt ur growth and also it can fuck ur hormones more easily etc.... but i dont think that matters so much..
IMO taking steroids is better at a younger age lol because at like 18-25 u r still studying and starting ur life kinda of... after u get older u need to focus some more on ur carreer, most of us will have families n stuff.. i want to enjoy roids while im young then when im older all i gotta do is like 1 cycle /year to keep my gains or something like that