What is the average waist size for a 12 year old?

It depends on your weight and height and other proportions.

When I was twelve, I had just started going through puberty and remember that my waist was about 20", because of the outfit that I wore for my Ballet performance. I am 15 now and my waist is 22".

The average for a 12 yearold is quite high, but for a healthy girl it is 20"-26".