I will state for the record that I am personally against piercing the ears, because it reminds me of eved nirtzeh. Sorry, but it just is an image I cannot get away from. That said, it is far easier IMO to pierce an infant’s ears, because they won’t pull at the earring studs as a toddler or older child might, and it will be easy to take care of the healing areas. On the other hand, I also feel this is something which is being done to the baby without her consent, and unlike a bris on boys, is NOT required by halacha, so I guess it bothers me a bit. As you can see, I am not so in favor of doing this.
But in the end, if you are going to do it anyway, do it in their infancy. Less traumatic for the baby. My granddaughter’s ears were pierced at around 6 months, and she did really well with them. I am still opposed, though, and my kids all know that, though all my daughters have pierced ears, which they did as teens.I opposed it, but I am not a dictator. They were old enough to make their own decisions at the time. The thought makes me ill.