The world is getting older. By 2030, the population that is over the age of 65 will rise by nearly 40 percent. By 2050, it will more than double. Aging populations present a host of political and social challenges. How can fewer workers fund a growing population of retirees, many of whom have been promised fat pensions? How can healthcare systems manage a big uptick in chronic diseases like heart disease, cancer, or Alzheimer's without going broke? Is immigration the answer? Or robots? These are just a few of the questions the next generation of political leaders will have to grapple with as our populations get older. Here's a look at which countries are going gray fastest.