What the stages of ballet for a beginner 12 year old?

It depends on the studio you are taking classes with, but most studios have beginning classes for various age levels. This is designed to make you feel more comfortable to be with people of your own age and level also a beginning 12 year old and a beginning 6 year old are very different in terms of their learning abilities as well as in the development in their bodies. I recommend going to the studio where you are planning to take class and asking them what class they would recommend for someone your age who is just starting out in ballet. Good Luck :)