The functioning of the reproductive system worsens over the years. That’s true for both men and women, but especially for women. One study has found that 29% of married women aged 40-44 are infertile, compared to just 7% of married women aged 20-24.
Declining fertility with age is built into the very workings of the female reproductive system. Your body only contains a certain number of eggs, and once they’re gone no more can be created. Some of these eggs will be ovulated and thus could be fertilized but this only accounts for about one in every 3000 of the eggs you were born with.
Most of your eggs will degenerate over time, in a process called atresia. Once you have no more viable eggs, you have reached menopause. As you approach this point, with declining numbers of eggs, fertilization will become more and more unlikely.
Menopause does not happen at a set time for every woman. This Male fertility shows a shallower and less complete decline with age. 70-year-old men have sometimes succeeded in fathering children, whereas a woman of this age would have absolutely no chance of becoming a mother. The decline in male fertility is partly caused by decreasing sperm production and worsening sperm quality over the years, but is just as much tied to general health problems which impact on fertility.
For example circulatory problems can be connected to impotence, incontinence can lead to retrograde ejaculation, and surgery in the pelvic region can have side-effects touching on the reproductive system. This link between general health and fertility is true of women as well as men. As with many other aspects of your health, if you take care of your body over the course of your life, it is less likely to let you down later on.
This link between age and infertility is one of the reasons for the apparent rise in infertility in recent decades. It isn’t so much that people are getting less fertile, more that they are waiting until their fertility declines before attempting to conceive.
To some extent this trend is counter-balanced by the increased ability of medicine to help us overcome the imperfections of our bodies but assisted reproduction techniques aren’t immune to the effects of age, either. Almost any fertility treatment will have a lower chance of success the older you are.