It's apparently not because it's entirely possible to have children well into your 30s. Sure risk factors can do things like "increase 10x" but it increases from like 0.02% to 0.2%.
Risks are much, much, much higher. Chromosome disorder in 40 year old mothers is 1 in 30. For 25 year olds it is 1 in 1250. So from .08% to 3.33%. Chance of miscarriage is 80% for a 40 year old mother. For a 35 year old, it is 20%.
