my 12 year old is asking to get her ears pierced should i

That depends on you, and your child.  Are you okay with her piercing her ears at 12 years?  When were yours pierced (assuming they are)?  Is she responsible enough to take care of them herself while they're healing?  There's a whole regime of cleaning and turning the posts daily until the holes have healed, and she could get a pretty nasty infection if she doesn't do it.  (I should know - I didn't clean regularly, and I got a horrific infection in one of my earlobes.  Hurt like heck, too!)  If you don't think she's ready, but you don't mind the idea, then use the piercing as a goal: set the parameters for her, very clearly, that she has to meet in order to get the piercings, and once she meets them, follow through and let her.