Not that long ago, retirement meant being put out to pasture, with long days punctuated by occasional games of golf and bridge. But today, with lengthening life expectancies and dwindling pensions, many Americans are looking to retirement as an opportunity to start a new business. "We've never before seen so many seniors who are this active and doing so many things,'' says Lisa Gundry, a professor of management at DePaul University's Kellstadt Graduate School, who has worked