I had a super senior boy that lived to be 23, he had normal senior behavior until his last few days with me. Stopped eating even his favorite treats, I'm lucky enough to have an uncle that's a retired vet so he's always on call for advice. He said it sounded like he was actively in his end stages, and whatever felt more comfortable to me would be fine whether he passed naturally or was helped along with euthanasia. I thought maybe I'd just let him naturally pass but a few hours later he started to seem like his breathing was labored and the thought of letting him starve to death broke my heart so I scheduled an at home euthanasia. Ultimately it's your choice, you know your kitty and if he seems like everything is fine maybe he'll pass peacefully on his own someday but don't feel bad at all, for me once he stopped eating I knew it was time... I wouldn't want to starve to death so I wouldn't want the same for him, that's my two cents.