Very close; the evil doctor (no matter how benign the vet really is!).

As long as the vet knows that it's still showing, I wouldn't worry about it.
I don't *think* it's age-related.  Otoh, haven't done a search on it,
either...

Cathy

I have had and seen many old cats but I have only seen the third eyelid
in sick cats. And in fact, none of the sick cats I saw it in were old.
You have only had him since August--maybe it just takes some time for it
to go away. You mentioned that he did not have proper nutrition for the
last years of his previous owner's life. I feel certain your vet would
have brought it up if he thought there were something else going on.
Although, he might have assumed that it was because of this recent
problem and not an ongoing thing.

I really don't know but the third eyelid showing concerned me.  From what
I have read, it is a sign of illness.  I suppose it could be old age, but
I am not an expert.   I  really can't remember details of individual cats
because there are so many.   As used in the group, TED means (I was told)
The Evil Doctor from the cat's point of view.   When I mention this to my
Vet, she said she got that impression from the cat's reaction.  Best
wishes and purrs that Jasper is OK>   MLB