NEW YORK ( TheStreet) -- A generation ago, it was rare for anyone to live to 100 years of age. But now when a couple reaches 65, there is a 10% chance that at least one of the partners will live to 100, according to recent data from the Society of Actuaries. There is a 1% chance that one partner will reach 107.The gains in life expectancy present a challenge for anyone who is developing a retirement plan. To maintain comfortable living standards, retirees may require savings that can cover expenses for four decades. But many planners fail to consider the new data on life expectancy. Instead, people in their 60s plan for retirements of 10 or 20 years. "Most consumers underestimate their life expectancies," says Angela DiCastri, assistant director of personal retirement markets for Northwestern Mutual Life Insurance. Part of the reason for the underestimates is that the numbers have been changing rapidly. For newborn American males, life expectancy has been increasing by two years every decade, according to the Society of Actuaries. Although the life expectancy of a man born in 1960 is 66, the figure climbed to 75 for someone born in 2010. Keep in mind that those figures represent median expectancies. Half of all recently born babies will live past 75. Also be aware that the life expectancy data reflect the outlook for newborns. Anyone who survives the hazards of childhood is likely to live much longer. A female who has reached 65 has a 50% chance of living to 88 and a 10% chance of reaching 98. How long will you live? To develop a forecast, you can check one of the many online calculators that have been developed by financial planners and insurance companies. To prepare an estimate on a typical Web site, you answer questions about your family history, diet, and exercise habits. One of the more intriguing calculators is run by Lyle Ungar, a professor of computer and information science at the University of Pennsylvania. To use his calculator, you answer a series of questions, including whether any of your relatives had heart disease and how many miles a year you drive.