Coach-- I would be far more concerned with fucking up your endocrine system with steroid use! Growth plate closure is definitely a very real problem, however, your endocrine system does not stop developing until ~24 years old!!
The endocrine system is responsible for natural hormone production and regulation (natural ability to produce testosterone, but not just that--estrogen, growth hormone, insulin, and many others!).
It is the very last system to reach maturity, and they could end up fucking themselves over for life by ensuring that their hormonal system never reaches its potential. For a young guy the most persuasive side effect is crappy hard ons and/or libido....for the rest of your life. There are a bunch of other thinga that can go completely wrong but that's probably the biggest card to play for a young guy's psyche.
Now, this is not to say that it WILL happen to everyone who does it in their mid teens. It's rather like russian roulette.. Might never happen, but do you really wanna be the one that loses?
ALSO-- it is worth noting that ONCE your endocrine system matures completely circa 22-24 years old these permanent side effects are pretty much not a concern if you start using steroids responsibly after this time. So this applies pretty much only to the really young crowd.