Why do 11-year-old girls get their period? why do they get it at that specific age?

Confusing question: 1st Menses will generally occur within 3 years of nipple growth, and may be earlier or later than age 11. Delay past 15 is rare. The body develops according to an interplay between the genes that direct the process, the hormones that emerge from the brain and adrenal glands. The exact timing for this process is unpredictable, but the sequence of events is consistent.