Okay, I can speak to this issue, as I used to work at an Earring Tree - a chain store that sells jewelry, and where you can get your ears pierced.

1. Cartilage has nothing to do with it. Piercing is done BELOW where the ear cartilage is at any age, unless you start doing multiple piercings, and go "up" the ear. Piercing is done in the soft fleshy part of the ear.

2. The store had a policy not do do anyone under the age of 4 IF there was only one employee on. With two employees, you can do both ears simultaniously, so even if the child freaks out (there's a noise from the "gun" as well as the pinch of the skin being pierced) both ears are done, no harm, no foul. So this policy didn't have physiological reasons, just psychological ones.

I pierced the ears of a teeny baby once, two weeks old, when I was the only one there. She was asleep, and her mom held her facing forward. I pierced one ear, she squawked a little in her sleep, then tilted her head to that side, so then I did the other. She was less trouble than many 20-something I did, who got themselves all worked up about it before coming in!

Metal allergies can happen at any age. My family tends toward them, so I never pierced my ears, nor did my mother or sister. I just never saw the point of it, I guess! (I did, though, have to wear magnetic earring to work at the Earring Tree, just to not freak out customers - "what? yours aren't done? Then it must REALLY hurt!")