My main wonder: Why did they limit the study to persons over 40 only?
Oh, well, I can always take solace in
"Why Most Published Research Findings Are False" John P. A. Ioannidis Published: August 30, 2005
http://journals.plos.org/plosmedicine/article?id=10.1371/jou...
and
"We have an epidemic of deeply flawed meta-analyses"
by the same author:
http://retractionwatch.com/2016/09/13/we-have-an-epidemic-of...
and the original paper "he Mass Production of Redundant, Misleading, and Conflicted Systematic Reviews and Meta-analyses" at:
https://www.milbank.org/quarterly/articles/mass-production-r...
1st sentence of introduction of the fine paper:
"Many countries have already increased their retirement ages by delaying the age at which people are eligible to start receiving pension payments."
Or less. If you're over 40, cognitive function declines if you work more than 25 hours a week (and declines more the more you exceed). Up to 25 hours a week, cognitive function increases with more work.
Not that increasing cognitive function is the be-all and end-all, but (a) it's certainly something to consider whether you're degrading your brain and (b) 25 hours probably also works well for other reasons and for other ends.
https://papers.ssrn.com/sol3/papers2.cfm?abstract_id=2737742