This study suggests that cognitive gaps between poor and middle class children show up as early as 9-24 months — and that income and the mother's education are the two biggest risk factors. The report recommends very early intervention for at-risk kids (starting at birth), as well as initiatives to support high school and college graduation programs for the parents an d professional development for at-home child care providers.
Halle, T., Forry, N., Hair, E., Perper, K., Wandner, L., Wessel, J., & Vick, J. (2009). Disparities in Early Learning and Development: Lessons from the Early Childhood Longitudinal Study — Birth Cohort (ECLS-B). Washington, DC: Child Trends.