Working longer will soon be necessary. Good thing it's healthy, too. April 30, 2006 At 68, Gerald Schwinn has never been busier. The retired energy consultant tutors kids in the Washington, D.C., public schools and works part-time at the public library and an art gallery. He'll keep working at least until 70, when he's eligible for maximum social security benefits. "As long as I can remember the Dewey decimal system, presumably I can stay at the library," he quips.Schwinn is in the vanguard of an increasingly gray-haired workforce that has the potential to transform the aging of the population from an economic burden into a net plus. Extending the normal working life span to 70, by raising the eligibility age for public and private pensions, for example, would add one-half of one percentage point to GDP growth annually over the next 20 years, and income levels would rise 11%, according to Goldman Sachs. Boston College's Center for Retirement Research says even unpleasant jobs keep older workers healthier. Because we're living longer, working longer makes sense. Five workers now support each senior age 65 and older. In 30 years or so, that ratio will drop to two-to-one unless more people defer or forgo retirement. Removing tax and benefit penalties that hit older workers would help keep them employed.