Since Down's syndrome is a chromosomal disorder caused by an error in cell division, the likelihood of such an error occuring increases with maternal age. This means that an older mother is more likely to give birth to a child with Down's sydrome than her a younger woman. However, overall, fewer older mothers have babies than younger ones.
According to the National Association for Down syndrome, "80% of babies born with Down syndrome are born to women younger than 35. The average maternal age is 28 years old." The likelihood of a woman under 30 years of age giving birth to a child with Down syndrome is less than 1:1000, but increases the older the woman gets (see chart below), with an incidence of about 1:60 at 42 years of age.
At 25 years old the risk ratio is 1 in 1400.
At 30 years old the risk ratio is 1 in 800.
At 35 years old the risk ratio is 1 in 380.
At 38 years old the risk ratio is 1 in 190.
At 40 years old the risk ratio is 1 in 110.
At 45 years old the risk ratio is 1 in 30.