"One idea is to raise the normal retirement age to, say, 70, but make it easier for older people to go on disability.... A streamlined alternative would be to base the normal retirement age on a worker's occupation -- raising it higher for deskbound jobs.... A sliding scale would seem suitable for an aging workforce, since getting old is one long process of sliding downhill in terms of ability to work.... [T]he share of people working above the traditional retirement age, while higher than in recent decades, remains lower in percentage terms than it was in the 1940s and 1950s."