manufacture dates on new helmet – how old is too old ?

Probably a system so they can recall if they have any issues? akin to serial or frame numbers.

^^^ I wondered about that, but when I asked Giro about the falling apart pads on the Montaro helmets they said they had been updated as a rolling change and couldn’t say if a new lid would have the old or new pads, which kind of negates that sensible theory.