Compared with the gene patterns of young brains, those of people aged from 40 to 70 were much more variable.
Some middle-aged individuals had 'young' genes while others were old before their time.
By age 40 some brains are aging quickly. This aging process is disabling for many professions. For workers who's job requires ongoing learning and analysis the aging of their brains means they're struggling long before retirement. Unless we figure a way to slow this process, delaying retirement means years spent bagging groceries -- not working in their original positions.