My daughter wanted her ears pierced when she was very young. I couldn't do it because I was afraid she would cry and I couldn't bear to see this. She kept after me and one day I was with my best friend and we had the kids out. She saw some people getting their ears pierced and kept after me to do hers. I told her I just couldn't because I didn't want to make her cry. She promised me she wouldn't, I still couldn't do it, so my friend had them done. She was only 3 at the time. I never had an issue with this and it was fine for me.
Now, on the other hand, she had her daughter's ears pierced when she was a tiny baby. In the end, she took out the earning and her ears closed up. When she turned 6 she wanted her ears pierced again so I asked my daughter what she thought. She didn't have a problem with this. I had them pierced a second time. For the first year, she wore the earrings and was happy with them. Then she started soccer and had to take them out. After this, it was hard to get her to wear them again.
She is now 16 and has finally decided to wear her earring again. This time she asked me to have the hoops put in her ears so they wouldn't come out. This is an earring for body piercing. I took her and had this done.
If I had to do it over again I think it is best to let the child make the decision when they are ready. I believe that when they are very young they don't really know if they want them or not. But the age of 12 I think they are now old enough to know what they want. I never had an issue with my daughter, but my granddaughter was another story. She was more of a sports person and had to always remove the earrings to play sports. In this case, she should have waited until later on to have her ears pierced but we allowed her to do it when she was only 6. I think 12 is a good age and you should be fine. She is now old enough to take care of them herself.