The urban area of Tokyo is by far the world's most populous, with 35 million people in 2005. About half the world's population lives in urban areas, a proportion likely to increase slightly, but most live in small cities under 500,000 rather than large agglomerations.
Mortality from HIV/AIDS has been reversing the trend toward longer life expectancies in some areas. Otherwise, for the most part reductions in child birth rates are leading to aging populations in both developing and developed nations.
Population is one of the key elements in estimation of future energy demand; the 37% population growth estimated between now and 2050, even while moderate on the scale of the 20th century, illuminates the level of energy demand growth we may reasonably expect.
Thanks to WorldChanging for referencing the report.