mine was 18 by the time we had to help her go, her kidneys were failing so there wasn't much we could do, she was all bones.
i do agree that loss of vision isn't serious enough to have him put down though, as long as he's not suffering. you all just have to make sure he doesn't get outside. and really he'll probably understand that he's disabled and won't want to go anyway. they're smart enough to know when to avoid danger usually.
what kind of food is he on? when my boyfriend's dog started to go downhill and her eyes were glazing and she wasn't seeing well, they changed her food and she had a huge turnaround, was seeing better and was more bright eyed and energetic.
i'd try a high quality food like nutro and make sure it's the one meant for seniors. there are supplements you can get also that are a liquid, you just add a spoonful to the food or water, it may help and doesn't cost much. older cats and dogs do need more care and better nutrition to keep them going, just like older people.
I'm singin' here to get rid of fear
Hope it disappears right here with the rain
But I know life is pain, not like a fairytale
Meaningless to pray, so just goin' on my way