How many years will it take for income per person to double?

AVERAGE incomes in developing economies are growing more quickly than at any previous time in history according to a recent report by the McKinsey Global Institute. It took more than 150 years from the start of Britain's industrial revolution for GDP per person (measured at purchasing-power parity) to double from $1,300 to $2,600. Around 120 years later, America, with a similar sized population, achieved the same feat in a third of the time. China did it in just twelve. South Korea's GDP per person has grown rapidly from the $2,600 mark around 1980 to stand at almost $32,000 per person now. Looking ahead using the IMF average of growth forecasts for 2011-16, GDP per person in both China and India could double from 2011 levels by the end of this decade. People in the developed economies will have to wait another quarter century to see their incomes double.
source:The Economist