I say 25. How long you're been training, what kind of shape you're in, diet, and body fat, none of that has anything to do with anything. Your body needs to fully mature first, otherwise you risk doing permanent damage.
I know this is not what you want to hear, but I'm sure if you start a few more threads asking the same questions, we will all change our answers, and once someone does tell you what you want to hear, your body will magically transform, and everything will be OK. The best thing you can do now is ask this same question at least 4 or 5 more times in different threads, then you should be good to go to run what ever you want. After all, if you can gain the exact same amount of muscle naturally, but with gear you can do in like a month or two faster, then all of the life long risks don't really matter, do they?