Adwoa, I risk sounding like a total lurker when I say... I always really like reading what you have to say. :D
I'm 17. All I've got are 00g lobes, and I researched the shit out of stretching before I even thought about ordering my first set of jewelry from BAF 'cause I really wanted to do it right. It really sickens me to see/hear about the kids at my school who just shove household objects or ill-fitting Hot Topic jewelry through their ears. Of course, we all know that teenagers are invincible, so if I try to give them advice, they roll their eyes at me. My own peers. It's bewildering.
I've thought about how I'd raise any kids I might have down the line, and I think that, rather than letting them put holes through their faces or permanent ink in their skin, I'd suggest they mess with crazy hairstyles as a creative outlet. Hair grows, and it can be cut. Piercings don't always close, and tattoos are terribly difficult to remove. When it comes time for them to get a job, if they've got rainbow-colored hair, they'll realize it's hindering them and hopefully dye it something more tame (or find a job that's accepting of that - hell, if my kid can do that, more power to him/her!!).
If they're really set on a piercing, though, I'd want them to do their research and wait at least a year or so to make sure they still want it. Then, if the kid doesn't take care of it and it all goes to shit, I'll make them take it out, talk to them about what happened, and consider letting them get it again in a year or so. No tattoos until they can get them on their own. And stretching... well, I'm not so sure about that yet, given my case 
This is, of course, everything I've thought of before I have ANY idea what it's like to have a kid. I hope that this is a good plan and that it works for both me and my family, but if life decides to teach me a lesson or two and I see that this is all wrong... Awesome. :3
Oi. I've totally digressed. In order to try to stay on topic... I don't really know what is "too" young. I'm not sure anyone can, since everyone matures at a slightly different rate. Though I guess I'd define "too young" as wanting something, whether it be a piercing or a ticket to the Hannah Montana concert, because your friends want it. "Mature" is when you start to figure out what you want for yourself - not to affect the way others see you; not to make a statement or raise a few eyebrows; but because it truly makes you happy regardless of anyone else's opinions, and because you feel like it's an important part of who you are.