To date, few studies have investigated the evolution of problem solving and general numeracy abilities during adulthood: skills that have obvious social importance. In this research, evolutions in adults' mental arithmetic skills were investigated using data from the IVQ 2004 French national survey, which tested 9,185 adults aged between 18 and 65. Whereas some of our results confirm previous work, others are more surprising. For example, participants with higher levels of education achieved higher scores than did participants with lower levels of education, and this was true at all ages; however, improvements with age in performance only were found for participants with low levels of education (first grade or lower). For higher levels of education, performances were stable. Our results suggest that lifespan mental arithmetic, a domain not affected by the Flynn effect, relies mainly on pragmatic cognition, and develops as a result of everyday activities, as well as through initial instruction. (Contains 2 tables and 7 figures.)