Work is too ambiguous and complex of a concept to directly state that 25 hours a week is optimum over a certain age.
It seems like this article is partially premised on research done on assembly line workers, who are, as far as I'm aware, actually working and doing physical labor for most of the time they're at work.
I wonder how well this applies to mental labor? Beyond that, as some people have already commented, white-collar jobs tend to involve a lot of downtime.
Left to my own devices I'm pretty sure I'd work a variable number of hours a week, going as high as 80, and probably averaging in the neighborhood of 60.
The tricky part for me is that breaking 25ish actually-productive hours a week isn't compatible with a steady 40ish hour a week job where I'm doing mostly the same thing every week.
Ideally I'd spend 40hrs studying math one week, 80hrs developing a game next week, 30hrs landscaping/gardening + 30hrs building furniture the next, 50 hours writing fiction the next, et c. It's doing more-or-less the same work every week, whether I'm in the mood for it or not, that places such a low upper bound on my total productivity.
It seems like this article is partially premised on research done on assembly line workers, who are, as far as I'm aware, actually working and doing physical labor for most of the time they're at work. I wonder how well this applies to mental labor? Beyond that, as some people have already commented, white-collar jobs tend to involve a lot of downtime.