How old is Sammy keyes?

Sammy Keyes is only 13 years old.

At the beginning of the series she thought she was 13 and then on her birthday in Sammy Keyes and the Psycho Kitty Queen, it turns out her mother had lied to everyone about her age and that she is really 12. So when she was supposed to be turning 14, she was actually turning 13.