There is no agreed upon age that a little girl should get her ears pierced. While it is a safe procedure for babies, there are a few things to keep in mind.
First, any time a needle pierces skin, there is a risk of introducing infection into the area. Second, small earrings can become a choking hazard if they are pulled out of the ear and end up in the mouth.
Also, some babies can be sensitive or even allergic to the metal in the earring posts. A good general rule is to wait until a later age when your daughter is old enough to take care of them herself. If you decide to do it earlier, make sure the procedure is done by a pediatrician or trained professional.