It is common for 11 year old girls to start growing hair in their armpits, and the timing of menstruation can vary.
For girls who are already 11 years old, how long it will take for menstruation to occur after the onset of armpit hair, there will be greater individual differences. The development of puberty in girls is usually due to the action of the hypothalamus on the pituitary gland, which secretes gonadotropins, prompting the ovaries to secrete estrogen, which leads to the development of armpit hair and breasts.
The appearance of secondary sexual characteristics, such as pubic hair, armpit hair, breast development and menstruation after the age of 9 is a normal phenomenon, but because of the different physical quality of each child, different living environment, there may be differences in the timing of the appearance of the symptoms, usually around the age of 12 years old, the girl will have menstruation.
There is no fixed relationship between armpit hair and menstruation. It is recommended to do a good job of psychological counseling and hygienic care for children.