There is some truth in "use it or lose it" when it comes to the brain. Mental decline is not an inevitable part of ageing and research suggests the more active your brain is, the better your memory will be. Research also shows that people who retire in their early 60s have a harder time with health than people who stay in the workforce. Part time workers also tend to function better on a daily basis, with fewer major diseases and better mental health.
Stress takes a toll on mental health and age-specific events such as retirement, bereavement or divorce are highly stressful. It’s not clear why work may help you stay sharp. It may simply be having something to do and being social. The financial boost of working longer can help reduce stress too.