Quoting Not tellin:" <blockquote><b>Quoting Karen Serratt:</b>" I have noticed a lot of women are having ... [snip!] ... with Down's syndrome. You understand why that is right? However, younger couples can have a child with DS based on genetics."
It's also important to note that while women over 40 have a higher risk, most children with DS are born to women under 40.
ETA: It also isn't JUST testing for DS, it's testing for neural tube defects and other chromosomal abnormalities such as Trisomy 18.