if someone is in pain, should we go for the kill!!

my cat is going to be 20 years old and for the past few months, its been meowing a lot lately :-(

it doen't even take a dump outside anymore, we had to put some litter in the bathroom, its tail is so dam skinny and she is starting to walk slower. i looked at her eyes and it looks like she's getting blind. she stays in bed most of the time and at night it meows even more as if she is about to die
my cat has been there even longer than most of my "bestest" friends. you think i should put her to sleep or what!!

we had other cats, but this cat is the one that stayed with us....my kitty cat is about to die !!! :-(