When does the period start at what age?

The average age is around age 12 but I believe some girls have been as young as 9 or as old as 16. It depends partly on genetics (when your mother & grandmother first got theirs) and partly on other factors. Girls who are very active in sports may start their periods later. So may girls who are excessively under or overweight.

The average age is around 12.  But like any average there are variatioins.  TFor the vast magority it will be 10-14 (and some will be below or above it !).   Records indicate that the age (for the 1st period) is steadily going down (over the ears) !.