That's the standard explanation (at least among people who don't buy the traditional magical theory of youth), and was my previous theory.

Actually, really, they're theories of different phenomena. People who don't do as much research simply because they're busy administrating aren't really "declining with age"; they just literally aren't spending as much time. The hypothesis I presented above was an attempt to explain the nature of specifically-age-related (but non-medical) intellectual decline, such as it exists.

The two cases can be distinguished by observing whether the senior professors return to pre-administration levels of productivity after they become emeriti.