Final answer:
Genetic factors are the most important factor in determining the age at which menstruation begins.
Step-by-step explanation:
The most important factor in determining the age at which menstruation begins is genetic factors. Genetic factors play a significant role in the timing of puberty and the onset of menstruation. While there are other factors that can influence the age of menarche, such as nutrition and physical activity, genetic factors have the strongest influence on when a girl will start her menstrual cycle.