When is it time to consider euthanasia for an elderly cat with Dementia?

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I was wondering if anyone in this position knows when’s best

we have a 17 and a half year rescue boy, he has for the last few months been showing signs of dementia still fine with his tray etc but now it seams he’s forgetting his had his food etc and also constantly meowing over night.

vet check is no pain etc just an old man but I’m wondering at what stage do we say it’s too much for him.

He sounds unhappy. I personally think that when you start asking yourself 'when is it time', then maybe it IS time.
Be kind to him and do what you know in your heart is the right thing (and believe me, I know how hard that is)

Things can only get worse for him sadly. Better a day too early... as they say.
Sorry you're having to face this. It's the hardest time for any pet owner.