2. The definition of menstruation a Menarche the onset of the first menses occurs about two years after the o pubert occurs between 13 and 15 vears of ago anovulatory for first two years2.The definition of menstruation ❑Menarche: the onset of the first menses occurs about two years after the onset of pubert occurs between 13 and 15 years of age anovulatory for first two years