Read other letters about this article
http://scicom.ucsc.edu/SciNotes/9701/full/features/cancer/home.html
http://www.tamu.edu/univrel/aggiedaily/news/stories/archive/111998-3.html
http://health.discovery.com/centers/pregnancy/americanbaby/pregnancybenefits.html
Actually if you google it, and it has been in the American Medical Journal, Scientific American .... pregnancy before 30 does decrease cancer rates in women by a significate rate.
The last link on the benefits, note the one about your periods - one of the causes of infertility is irregular menstrual cycles. Pregnancy frequently improves and restores normal menstrual cycles - fertility still goes down with age, but if you've already had a child, you've extended your regular menstrual cycles out for a few years, also infertility can be caused by a lack of fertility hormones (estrogen, progesterone, folllicle stimlating, luteinizing, gonadotropin) these decrease with age - but after a pregnancy they go back to normal(or are increased by the pregnancy) - atleast for awhile - so you've removed two possible causes of infertility by having a child. Having a child before 30 improves the odds of having a child later. That doesn't mean forever - everyones fertility is still declining rapidly after 30, its just a matter of odds. (And that 30 is not a hard and fast rule, it depends on genetics and when women in your family start menopause, it could be earlier or later)