You can't have read my reference then. And I said overall debt to GDP - the reference explains how they get to 60% and how they can manage an ageing population. You might want to consider there are cost to a rapidly growing population too such as huge infrastructure bill, loss of productivity due to congestion, loss of amenity and strain on the environment. Also the large government debt the US has had for a long time has not had any adverse effects and some economists question whether government debt really matters that much.