You are correct, you are too young to start using AAS. At your age you have enough test in your system naturally to build a substancial amount of muscle. The damage that could be done at a young age may not be reversable and a lifetime of HRT may be needed. Wait AT LEAST 3 years with solid training and diet etc.
Keep reading and researching so when you take the plunge it is well thought out and correctly executed. As far as detection times it is going to be determined by the actual steroid used. Some have short detection times such as test suspension, while others like deca can be detected over a year after last injection.
I beleive there is a thread that has them all outlined, do a search on detection times.