From the Center on Longevity site:
Over the next 30 years, the US population age 65+ will double from 40 million to 80 million, and the share of old people will increase from 13% to 20%. By the time the last baby boomer turns 65 in 2029, one in five Americans will be age 65 or older. By 2032, there will be more people age 65 or older than children under 15.
United States Demographics. (n.d.). . Retrieved May 11, 2013, from http://longevity3.stanford.edu/united-states-demographics/