Why are women 35 years old and older more prone to getting a Down syndrome child?

Egg injury. A newborn girl is born with all the eggs she will ever have, and their quality reflects exposures to toxic agents throughout life. Men get more of a pass on this because they make millions of sperm and the healthiest sperm swim faster and do the fertilizing. We know that over 35,the chances of DS begin to rise, reaching 1/5 by age 55.
Relative age. It is generally assumed that the conception machinery is older and more prone to trisomy. Each time a cell divides in growth of the embryo the chromosomes are normally divided equally between the daughter cells. In down's an early cell receives 3 number 21 chromosomes (trisomy). There is also an inherited type but that is rarer and usually will appear in the offspring of younger mothers.