Conventional wisdom tells us that cognitive skills continue developing until people reach their early 30s and then begin a long fall. However, that conclusion does not come from following individuals as they age. Instead, it comes from comparing the math and reading skills of individuals of different ages at a single point in time.

My colleagues and I find that skills typically rise until the 40s, after which reading skills gently fall and math skills more steeply. Even here, however, the story is not so simple. These averages mask the fact that any decline is closely tied to how much the skills are used. Simply put, people who read and do math on a regular basis hold on to those skills at least into their 60s.