[xlayn]

To me seems logical: As we get older more and more energy would be used on preservation (read it as fix damage and less efficient process as result of age), therefore shrinking/eliminating everything not being used it's necessary.

Edit: TL;DR: From the PDF conslussions:

it is found that working hours up to 25–30 hours per week have a positive impact on cognition for males depending on the measure and up to 22–27 hours for females. After that, working hours have a negative impact on cognitive functioning.

[Noseshine]

    > To me seems logical: As we get older...

Since the paper only looked at people above 40, I don't see how you can make any such conclusion. There is nothing about younger people. They may do better - they may also be the same or worse. They were not even included.

[xlayn]

Nice catch, there are two parts in trying to support my theory, first included in my post:

As we get older more and more energy would be used on preservation (read it as fix damage and less efficient process as result of age), therefore shrinking/eliminating everything not being used it's necessary.

The second is an entry on how our bodies are machines oriented to try to avoid wasting energy... or better said preserving it... for that part my canonical reference would be the Algernon argument:

http://www.gwern.net/Drug%20heuristics