the structure does degrade with age.
This isn’t true, although I suppose given enough time, anything will degrade.
The reality is that salt water and excessive heat will help deteriorate the foam but it’s not affected by UV, or very little else come to that. Obviously cyanoacrylic glues and the like eat into it, but with normal day to day wear and with a little regular cleaning, a helmet should last for as long as you fancy a change. Impacts notwithstanding, obviously.
I have a link somewhere but I’ve recently changed computers and my imported bookmarks have no symbols attached so aren’t easy to identify.
There are a lot of myths in the cycling world – helmet degradation through normal use is one of them.