Why would maternal age be associated with an increase in errors that lead to down syndrome?

Time = injury: A man gets a bit of a pass on defects since sperm renew constantly from stem cells.They are all young and used or lost before they get old.A woman's eggs are present at her birth.They are bombarded by background radiation, the woman's illnesses and diet,her good habits or bad. In the 20's the eggs have a low risk of DS but gradually increase to 1/20 by age 50.Accumulated damage is the theory.