Their output in 2010 was less than half the G7's GDP.
The projections of the Paris-based OECD, a club of indutrial democracies, are based on 2005 purchasing power parities (PPP).
At market exchange rates, it will take emerging markets a bit longer to seize the crown - for example, Goldman Sachs reckons the BRICs quartet of Brazil, Russia, India and China will overtake the G7 by 2037.
Asa Johansson, one of the authors of 'Looking to 2060: long-term global growth prospects', said the report presented a hypothetical scenario rather than a firm projection.
Nevertheless, she said the extent of the expected shift in economic power away from developed countries was striking.
Measured in 2005 PPPs, China and India will account for 28 percent and 11 percent respectively of the output of 42 major economies by 2030, compared with 18 percent for the United States, 12 percent for the euro zone and 4 percent for Japan.