Final answer:
The hormone necessary to support a girl's menstrual cycle starting as early as age 10 is estrogen. It is essential for sexual maturation, menstrual cycle regulation, and the preparation of the uterus for potential pregnancy.
Step-by-step explanation:
In typical development, the production of estrogen, which is necessary to support a girl's menstrual cycle, begins as early as age 10. Estrogen, produced by the ovaries, plays an essential role in the maturation of the female reproductive system during puberty. It stimulates the growth of female reproductive organs, as well as the development of secondary sexual characteristics. Furthermore, estrogen is responsible for the regulation of the menstrual cycle and the preparation of the endometrial lining for potential implantation. The onset of puberty in girls is marked by the pituitary gland signaling the ovaries to produce this crucial hormone.