What should be the average age to start masturbation (girls)?

No answer: There is no answer at all to this question. Some girls never masturbate. Some toddlers rub themselves in private areas at age 2 because they find out it feels good. Sexual development has a wide range in girls as well, with some girls now developing at age 7-8, and others not until mid-teen years. Just no one age that is the right or wrong answer to this question.

There is no average: Some girls start as babies when they discover it feels nice to rub their clitorises sometimes with their fingers sometimes by rubbing against a chair or table leg. Certainly majority of normal healthy post pubertal girls have discovered the pleasures of masturbation before they are adults. Some girls are inhibited because they have been told that it is sinful, unhealthy or dirty.