What age do most girls have their first period?

Varies, average 12yo. The start of cycles is called menarche. Girls experience menarche at different ages. Factors such as race, stress, diet, genetics and weight (bmi) are all factors involved. The average age of menarche is about 12.5 years in the usa. If no cycle beyond 16 years of age, the delay is referred to as primary amenorrhea. This requires an evaluation of anatomy as well as hormonal blood tests.