What is a good age to allow kids to sit in the front seat?

My little girl was in the front seat of my car from the day she was born. However I didn't have an airbag in my car. Now I have a car with an airbag but she sits in the back as she has a little sister and they need to be treated the same. I do let them sit in the front if they are on there own, I didn't realise the airbag could harm them.

In California, I believe the age to sit in the front seat is 12. I know some states will allow you to put your child in his/her car seat in the front seat if the airbag is disabled. I think it varies from state to state, but that's what I know.