Around the world, populations are growing older. But is that because people are living longer? Or could it be that there are fewer younger people to dilute the demographic pool? And what about aging itself -- when exactly is 'old' these days?Sarah Harper, an Oxford University professor of gerontology and director of the Oxford Institute of Population Ageing, grapples with these sorts of questions every day, asking how these changes will affect relationships, labor, migration, and even the environment. And while she presents the questions as challenges, she's not arguing these challenges need end in tears."In the last 25 years," she notes in this podcast, "this debate has moved around from the problem of an aging society to the challenge of the society of an aging society. And now people talk about the opportunity."Harper started her career as a news reporter for the BBC before training at the University of Chicago's center of Demography and Economics. Her postdoc career took her to China and the Pacific Rim, and she was the first holder of the International Chair in Old Age Financial Security established at the University of Malaya in 2009. She also is involved with a number of demographic and aging-related projects, such as being co-principal investigator for the Oxford Global Ageing Study and leading The Clore Population-Environment Interactions Programme.