It is closely related to the increased rates of complications and diseases and problems as we age. Older women are simply more likely to have a medical problem to begin with. Pregnancy is a major strain on the heart and blood vessels, and in turn, on all the organs in the body. Nearly any pre-existing condition will be worsened during pregnancy and make a more difficult pregnancy. Like many age-related correlations in medicine, the risk depends on things that happen to us over time, not just age itself.
Older mothers are more likely to have had other children.
Some of the complications, like the placement of the placenta over the cervix, are more likely in women who have had many prior pregnancies. An older mom is one who has had more time to have had gynecological procedures, problems, and diseases that may change her organs in ways that make complications more likely. Older women tend to release more than one egg a month, leading to higher rates of twins. Twin pregnancies increase the risk of complications, in younger and older moms. In short, it is more important to look at the woman's medical history, and the pregnancy itself, rather than at the woman's age.