I had my daugther's ears pierced when she was 3 month old. As soon as she got her first shots..
In my opinion is better to do it when they are small, because they won't remember the few seconds of pain, and won't be traumatized by that...
Also the healing process is allot faster because they won't be touching their ears (avoiding a posibble infection) and you can clean them regularly
If you are going to do it, ask the person in charge to get an other person and do both ears at the same time, is better than one at the time...