The "decline" is based on that in the US younger people do worse than older people when compared to other countries. Think of the study as a core sample.
"Some countries are making progress from generation to generation. But in the United States, as in Britain, the literacy and numeracy skills of young people coming into the labor market are no better than those who are about to retire. Americans who are 55 to 65 perform about average in literacy skills, but young Americans rank the lowest among their peers in the countries surveyed. The problem is not so much that the United States has gotten worse, but that it stood still on indicators like high school graduation rates while its foreign competitors rushed forward. Beginning in the 1970s, other developed nations recognized that the new economy would produce few jobs for workers with mediocre skills."
"Some countries are making progress from generation to generation. But in the United States, as in Britain, the literacy and numeracy skills of young people coming into the labor market are no better than those who are about to retire. Americans who are 55 to 65 perform about average in literacy skills, but young Americans rank the lowest among their peers in the countries surveyed. The problem is not so much that the United States has gotten worse, but that it stood still on indicators like high school graduation rates while its foreign competitors rushed forward. Beginning in the 1970s, other developed nations recognized that the new economy would produce few jobs for workers with mediocre skills."
http://www.nytimes.com/2013/10/23/opinion/the-united-states-...