What age is it okay to pierce a little girl's ears?

My mom said I got mine pierced when I was a baby (6 months). I have 2 girls ages 7 and 4. My husband and I agreed that we would wait till they asked. The oldest asked so I took her to get them done. Then the youngest asked at the store. So I went ahead and got it done too. Husband was mad and said she was too young. I said she asked so I said okay.
What age is it okay to pierce a little girl's ears?
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