Because teens are not done growing and need more constant nutrition than an adult. If it's ok for a teen to fast then it should surely be ok for an infant to fast, right? Probably not so much.
Your bodies are changing so much every day. Your hormones change. Your bones grow. Your muscles grow. Your brains grow. You can't starve those systems or you could likely "stunt your growth", pun intended. It's a well known fact that people continue to grow until they hit somewhere between 18 and 21 (typically). Until then, I think fasting is a bad idea.
And the reason I don't think anyone under the age of 18 should be here is because teens are more prone to eating disorders (maybe not statistically, but that's how it appears) - maybe not guys, but most certainly girls. Looking at me now you'd know I was never one of those girls (hehehe).
It's just not a good idea in my mind to be touting diet advice to a teen because most times the advice is bad or is taken out of context and causes problems.