It varies. The average age of girls at their first period is 12.6 years, with a range of 9-15 years.
9-17 year old. There is a wide variation but 9-17 ( even 18) is the range. More and more girls are menstruating earlier. As an average traditionally is 11 or 12 years old. If in doubt take her to the pediatrician or gynecologist.
Depends on kid. There is some variability in the timing of menses.It generally occurs ~1yr after the growth spirt, >30% by 12, 90%by 13. Other features of sexual development will be well in place by that point. If you have a growth chart, timing can be estimated as the age the girl reaches 2-2.5 inches from her final progected height. They often have straw colored discharge in the months leading up to it.