I believe that 40 is the age for screening and the reason being that younger women do have denser breast tissue and mammograms just aren't that useful for younger women.
Here is what I found on the American Cancer Society website:
"Women age 40 and older should have a screening mammogram every year, and should continue to do so for as long as they are in good health. "
http://www.cancer.org/docroot/CRI/co...ocedures_5.asp