Ah, he is a lovely boy , 21 is a fabulous landmark birthday for a cat !
Our old cat was 17 (still jumping up on the NDN 6' fence but we had to put a dustbin beside it to help her and she didn't do it with grace or style )
NDN cat was 21 , they found him as a tiny kitten.
One of the reasons I chose to have DCat PTS (she was going into all the old age changes that happen with cats) was she wasn't eating and was "struggling" .
NDN cat had been PTS 6 months before so she lost her bestie .
The Vet said her thyroid levels were high (she had regular blood tests in her older years ) with the risks involved .
She went outdoors a lot and I didn't want her wandering off to die , possibly suffering ?
She wouldn't have tolerated being kept in.
Our Vet said we were making the right decision. She had a great life with us and I wanted her to have as good a death as possible . Which she did .
. Your cat sounds amazing .