Why is the risk for Down syndrome higher when the pregnant woman is older ?

As a woman is born with all her egg cells, the risk of a chromosome anomaly in the egg cell increases with her age.

When a pregnant woman is 30 yrs, the risk on Down syndrome is 1 on 1000 (0.1 %); at the age of 40 yrs the risk is 1 on 100 (1 %), and at the age of 50 yrs the risk becomes 1 on 10 (10 %). The father continuously produces sperm cells, and therefore his age is not important.