It depends on what it says on it's label. If it's a steroid associated with horses the first thing that comes to my mind is boldenone, wich, by the way, is an aromatizable steroid, though it doesn't convert into estrogen, because it's not a testosterone, it still is aromatased by the aromatase enzyme into the estrogen analogue to that substance (the same way it dosen't convert into DHT (dihydrotesstosterone) either but still converts into DHB (dihydroboldenone)by the same enzyme that makes DHT).
As far as the question you've made, I agree with what's been said before about aromatizable steroids stunt growth and non-aromatizable don't, and I also agree with the fact that if you don't have your growth plates closed then you should wait for a while till use steroids (think that you may even get taller and wider on shoulders, wich can make any mass added even more proportioned and pronounced ehehe).