A study involving 180 13-year-olds who had been assessed every year since kindergarten has found that their understanding of the number system in first grade predicted functional numeracy more than six years later, but skill at using counting procedures to solve arithmetic problems did not. Researchers controlled for intelligence, working memory, in-class attentive behavior, mathematical achievement, demographic and other factors.
Number system knowledge includes an understanding of the relative magnitude of numerals, their ordering, and the ability to combine and decompose them into smaller and larger numerals and to use this knowledge to solve arithmetic problems.
Tests for functional numeracy were chosen based on labor economic studies of employability, wages, and related outcomes in adulthood. One in five adults in the U.S. lacks the math skills expected of an eighth grader.
Full text available at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0054651
 Adolescents’ Functional Numeracy Is Predicted by Their School Entry Number System Knowledge. PLoS ONE. 8(1),(2013).