It depends on what age they hit puberty, it can range from 9-16
How Old Do Girls Get Armpit Hair?
That depends on when they hit puberty . Some at 9 some at 11 years old .
13 year's girl should get armpit hair
You can get it at 9 because I have it and it itches
Anytime between 9 to 18 years and that is a horrible question to ask