If you want to live to be 100, make sure that your mother is under 25. That's what scientists at the University of Chicago say after analysing historical data on 198 centenarians born in the United States between 1890 and 1893. They found that first-born children were 1.7 times more likely to live to 100. The reason, they surmise, is the age of the mother (but not of the father). Why? Perhaps a younger woman's eggs are more youthful; perhaps a younger woman has not been exposed to as many diseases and infections and therefore provides a healthier environment in the womb. The study was presented to the Gerontological Society of America's annual meeting.