why does the reproductive age of a woman start menarche and end at menopause?

The first menstrual flow begins at puberty and is termed menarche. At 45 to 50 years of age, the menstrual cycle stops.
Stoppage of menstruation is termed menopause.

In this period only, during ovulation period the egg would get released and can fertilize with a sperm (if copulation occurs in this period) and the chances of being pregnant and give birth to a baby is high. That is why the period between menarche and menopause is called the reproductive age.