Women in their forties, fifties, and even sixties can all achieve very high pregnancy rates with egg donation. This is excellent proof that fertility decreases with age due to aging of eggs and chromosomes. Studies that have removed eggs from the ovaries of older women have shown that a large percentage of them have abnormal chromosomes. Of those that do, many will be lost to miscarriage. A few that remain as viable pregnancies, will end up as a Down's Syndrome or a related chromosomal problem.
Every human being is different.
Not all women will develop abnormalities in their eggs' chromosomes at the same age. The great majority of women will not be able to become pregnant at age 46. On the other hand, even some younger women may have a rapid decline in the ability of their eggs to produce a pregnancy. Such women often have an early menopause. FSH stands for Follicle Stimulating Hormone.