There is no best age really. However, I firmly believe that it is best for the body if you wait until your mid to late twenties. If you are competing in body building or powerlifting, this is somewhat of an unreal expectation though.
If you do decide to go earlier than that, it would be beneficial to check in with your endocrinologist and get all of your stuff checked out. Using gear before yourgrowth plates are fused will cause a premature closure of these plates. Which means you are robbing yourself of additional height. Since I am a whopping 5'9", I would damn sure recommend not stopping that too soon. lol
Read and learn bud. You have quite a bit of time of reading and learning before you need to jump on gear.
Aside from the obvious medical sides that can result from using too early, there are other things as well. If you dedicate yourself to learning about training and diet, you can gain as fast right now as you might on gear...and do it in a much healthier fashion.
What you learn, you don't lose. You will continue to apply this knowledge until you hit the wall with gains..where no changes in diet or training make a difference. then when you add gear to the mix, you already have your diet and training figured out. gear just gives you that extra bump over the hard spots.
Gear is NOT a crutch for a bad diet or a band aid for poor training. so don't use it as such. You are simply short changing yourself if you do.
There are psychological sides to deal with as well. for some of us, once we use gear...we feel we can't grow without it (I suffer from this mental mix up myself). It can become so severe that you won't even train unless you are on gear..I know someone like that. SAD