How is population change transforming our world? Think of a python swallowing a pig: a big bulge makes its way slowly down the snake from the head end to the other end. That's a bit like what's happened to the UK demographically. The baby boom generation - which has changed Britain politically, culturally and economically - is now retiring. That means a large bulge of pensioners with big implications for the generations that come behind them. Other advanced economies face a similar challenge and emerging economies - most notably China - will be dealing with an ageing bulge themselves soon. But in Africa, the bulge is at the other end. A very young generation is about to make its way through the snake. Former government minister David Willetts, now executive chair of the Resolution Foundation, wrestles with this python of population change. What will these challenges of both ageing and very young populations mean for the world? What are the implications for future migration patterns, for geopolitics and for global economic growth? This programme is part of a special week of programmes for the first week of 2017, examining major forces which are changing the world around us. Producer: Rob Walker.