A list is nice, but just a band-aid.
What is needed is a clear UI-indicator, which units are affected by a general. As generals are now area-specific and many of them can be around, it is simply too annoying to always counter-check by clicking on the general's unit and verify his "beneficial age". (That there is not even a Civipedia entry about a unit's age is just the icing on the cake ...)
There is even already an according visual effect implemented in the game: the white district highllight for other great people. Just highlight all affected units in the same way, if a general is selected.
An easy and very useful solution, I think.
--
(It is such a shame, that Civ6 misses out many of equally easy solutions, imo. Another one would be the sending of civilian units via direct map click. Why is it possible to directly transfer a trader to a new city, but not to start a trade route by simply clicking on the target city? Why can't a spy be directly placed, but always needs the "side bar list"?
I understand that the side bar grants additional information about the target city. But sometimes, I am simply not interested in them, but quickly want to move the unit: The trader to build a road, the spy to get vision to enemy territory.)