Actually, I'm starting to suspect it is. (Well, not literally "just", of course.)

My current theory is that people who do great work in their 20s don't do so later mainly because: (1) their status is already secure, and they don't have to work as hard to maintain it; and (2) continuing to work on the highest level would require them to study the ideas of (and thereby subordinate themselves to) lower-status younger folk.

This theory came to me when I observed that some older academics appeared to have lost their intellectual curiosity, not just their physical stamina (or whatever variable people think it is that causes the [alleged] phenomenon).

That said, my comment was actually about why we don't see people do great work later after failing to do so in their 20s, not why we do see people who do great work in their 20s fail to do so later. The point was that, after some had done great work early, having-done-great-work-early became a coveted, even necessary, status signal.