In another reverse of a historic trend, childless, working-age adults are also now as likely to be poor as anyone else - something the IFS pinned on weak earnings growth increasing the numbers of in-work poor.
Mr Phillips said: "The face of poverty has become much younger during recent decades.
"This is in many ways a triumph of social policy. But these figures also confirm that it is young people who have suffered most as a result of the recent recession and who are now at risk of falling further behind.
"It is important that policymakers and politicians understand these profound changes to patterns of low incomes and respond accordingly."
The conclusions are contained in the Living Standards, Poverty And Inequality In The UK report, funded by the Joseph Rowntree Foundation and based on the Government's Households Below Average Income data published earlier this week.