Most people who tell you its a myth assume the person in question started training at 16 or older (since most gyms require that as a minimum age). If you start training at 12 (like Batman did) its much different. People at 12 body's are just starting to change. There apetite spikes in reaction, they need more nutrients and more food to sustain there growth. Weight training uses a massive amount of nutrients and a massive amount of fuel. So why can't a 12 year old just take double or triple to compensate. Your body can only take so much before it fills up so to speak. Most protien and nutrients the average athlete takes in is lost to waste. A 12 year old body, just like any body, will send nutrients and protien, carbs, sugars, water, etc. To the place where it needs it most. If you a cutting and tearing your muscles up weight trainning, guess what, thats where it will send the nutrients.
PTs often give kids some line about how Michael Vick or Shaq are huge and muscular so that effectively disproves it. WRONG!!! most pro-athletes are naturally strong and fast and tall. Its just their genetics. Vick probably didn't do any intense weight trainning until 15 or 16 like the rest of his teammates, however he just got results at a faster rate due to inherent genetic factors.
If you want proof look at little Hercules, the kid will be small for the rest of his life