As I've mentioned before on this forum we have over 65 and a few 70 year old F/O's a couple of whom are not faring too well but hang in there anyway for some unknown reason.
They don't go back to another type to stay in the left seat because they'll fail the type rating.
If the grey matter between their ears is decayed enough to prevent them doing a type rating it probably means that the 'experience' that's always mentioned has probably decayed as well.
I've got personal experience of this decay, they should give it a miss.
I'd cringe to the nth degree if what is said about these guys was said about me.