That people sometimes err when recalling their age is not uncommon. (I have even been guilty of it myself!) The problem is more acute in historical settings, and even in developing countries today: individuals with limited access to education are less able to calculate their age. An observed tendency is for innumerate people to round their age to the nearest five or ten. This gives rise to a phenomenon called age heaping, whereby data reveal sharp peaks of individuals aged (for example) 30, relative to either 29 or 31.
Age heaping is a type of measurement error, for which different adjustment techniques can be employed. But age heaping has also come to be used as evidence: in the absence of data on education levels, changes in the incidence of age heaping over time can be used to infer changes in education. The logic of this is attractive: all else being equal, a decline in age heaping suggests a rise in numeracy. But all else is not always equal.
In a new working paper, McLaughlin, Colvin and Henderson offer a warning on the perils of overinterpreting the significance of age heaping. Using census data from nineteenth-century Ireland, they show how age heaping can give an inaccurate picture of educational attainment.