Estimates of the age of children are often used uncritically in anthropometric measures. This study shows that even with construction of calendars for use of determination of age, substantial training, a careful follow-up in the field by research assistants, and control of all questionnaires immediately after the interviews of the caretakers and weighing of the children, errors remain in estimating the age of children. Such errors may affect the results substantially, leading to errors in the estimation of age-based measures of nutritional status. In the case of Northern Mali, the effect was most likely an underestimation of malnutrition by perhaps as much as 10 to 30 percentage points. The biases in age estimation in many cases are not constant across subgroups of a population. Therefore age estimation problems may lead to wrong decisions regarding policy formulation, planning of development programs and activities, identification of target groups, and, in particular, evaluation of programs and activities. In situations where age has to be estimated, anthropometric measurements that are less influenced by errors in age estimation are recommended.