The aging of mothers, from the 20-25 age range to the 35-40 age range, means that processes affected by aging change. Nondisjunction is more likely in older mothers, leading to a greater likelihood of Down syndrome in children conceived from those nondisjunct eggs.
Work Step by Step
The only change in basic description from one group to another is age, so that must drive the change in outcome (frequency of Down syndrome). What exactly about aging is causing this is not obvious from the description: it could have to do with the functioning of the mother's body affecting how often meiosis completes successfully, or it might have to do with changes accumulated by the eggs over the additional 5 years of maternal life.