Every woman I know who had a DS baby had them when they were in their 20's. I know that the belief is that older women give birth to DS babies, but I have never seen it happen.
http://www.nas.com/downsyn/welcome.html
"About 80% of babies with Down Syndrome are born to mothers under the age of 35. About 1 in 400 babies born to women over 35 have Down Syndrome"
Robyn