The U.S. population is getting older as the “age wave” of baby boomers nears retirement. Will their 60s, 70s and 80s be happy years, marked by prosperity, good health and fulfilling activities? There’s every chance of that, according to two keynote speakers at Wharton’s 2007 Economic Summit: Wharton finance professor Jeremy Siegel and Michael Milken, the legendary junk-bond king who now heads The Milken Institute, a non-partisan think tank. The developing world may step up to fund boomers’ later years by purchasing the stocks and bonds in their retirement accounts, Siegel says. And Milken envisions prosperous, active lives for 80-somethings who feel like they are 60. But this future isn’t guaranteed. Protectionism could undermine older Americans’ prosperity, Siegel warns, while Milken cautions that medical advances could be offset by unhealthy lifestyles and a lack of investment in “human capital” that would cause the U.S. to fall behind.…