This is from Wikipedia, but it falls squarely in line with everything I've read on the subject previously *& is fully footnoted: http://en.wikipedia.org/wiki/Down_syndrome
Maternal age influences the chances of conceiving a baby with Down syndrome. At maternal age 20 to 24, the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19. Although the probability increases with maternal age, 80% of children with Down syndrome are born to women under the age of 35, reflecting the overall fertility of that age group. Recent data also suggest that paternal age, especially beyond 42, also increases the risk of Down syndrome manifesting.
So: The individual's chances are much greater when the woman is over 35, going up every year, but most Down Syndrome babies are born to young women because the vast majority of all babies are born to younger women.
P.S. It's called Down Syndrome in the United States, Down's in England. National Down Syndrome Society: http://www.ndss.org/
Now, if you find the statistics for Down’s children born to a 20-year-old, that might be relevant.