I got my earlobes pierced when I was 8, then my left helix (upper cartilage of the ear) done when I turned 13, then just a few weeks ago I got a second set of earlobe piercings (I'm 20 now)
I love all my piercings! Definitely not very many and not so adventurous, but they make me happy and look aaaawesome!
They all hurt, for sure. I got my first earlobe piercings done at claire's with a gun (I didn't KNOWWW!!) and it was no fun.. but it was a long time ago so who cares
The helix piercing was painful, but not awful if I remember right. Took quite some time to heal, as cartilage does, but it's awesome now, and definitely my favorite piercing.
The new ear piercings are still tender.. they got a little infected about a week and a half after I got them, so I'm trying hard to take good care of them. They're still a little screwy, though -.- no fun! But it's just the earlobes so I'm confident it won't take long for them to get better with proper care.
I think piercings are okay for teenagers (for the most part) but it gets really touchy when you're talking about younger kids.
I know I wanted my ears done when I was 8 and never regretted it, and indeed I cursed my parents for not getting it done when I was a baby. But now, I don't really think it's appropriate to pierce infants. While earlobe piercings are perfectly acceptable in society for girls, I think that decision should be made when they're at least a little older. That's just my thought on it, of course.
When it comes to more obvious piercings, especially facial piercings, that's definitely something that should be waited for. I think 18 is a fairly decent cutoff for it, but I'm sure there are some people who have gotten facial piercings younger than that who are totally fine with it. It's just a little bit more of a commitment to make than piercings on the ears or somewhere else where they're not necessarily "obtrusive."
Totally rambling.. piercings are pretty subjective, and need to be decided on with care. They're less permanent than tattoos, but are still an alteration to the body and should be 100% wanted!