Generally, men are fertile throughout their whole life, but a woman's fertility is limited. The childbearing age range in women declines eventually from puberty to menopause, usually between ages 15-49. The optimum time period for a woman to get pregnant is between ages 18-28.
As stated earlier, female fertility drops substantially from ages 35-37 onwards. Thus, while a 20-year-old woman has a 25% chance per menstrual cycle to get pregnant naturally, this percentage is reduced to 5% by age 40 onwards.