Some nationalities believe it has to be done before the baby is 6 months old - that's why you see so many small babies with pierced ears.
I believe a person has to be old enough to decide for themselves if they want their bodies 'altered'. But that said, ear piercing is quite innocent isn't it? I had mine done when I was about 11, but I never asked before that, and once I asked my parents had it done.
In my opinion six years is fine if that is what she wants. Does her friends have their ears pierced? And it is not really that painful, is it? She must just understand the aftercare that goes with it.
But again, this is just my opinion. Do what you feel comfortable with.
I got mine done at 14, i wanted them done sooner but my mother said i had to wait until i was old enough to properly take care of them as well...i was so happy once i got them done! i would wait if i were you, your DD will have plenty of time in her life for all that!!