• Notes by Acemoglu and Johnson (2007)

Age heaping pattern can be used as a proxy for numeracy at region level, especially when you work with historical data

• A’Hearn, B., Baten, J., & Crayen, D. (2009). Quantifying quantitative literacy: Age heaping and the history of human capital. The Journal of Economic History, 69(3), 783-808.
• A blog post