Although your chances of chromosomal abnormalities increase as the maternal age increases, like Downs, it's still acceptably low.
For a woman age 35, chance of a Downs child is 0.3%. Age 40, 1%. Age 45 and older, 4%. So if you are 45, you still have a 96% chance of NOT having a Downs child. Most children with Downs syndrome are born to younger mothers, simply b/c more younger mothers have children than older mothers.