We had our cat put to sleep years ago. It was a horrible choice to have to make, but it was the right decision at the time.
He was 13 -- not tremendously old for a well-cared for indoor cat, but he was definitely not the cat he'd been in his youth. He was declawed on all four feet, and he had some arthritus; it was obvious that sometimes -- not all the time, just sometimes -- he hurt a little when he walked. He could no longer jump up on to the sofa or the bed; instead, he'd come up and cry to be picked up. He was much slower than he had been, and he just generally didn't seem to enjoy life as much.
The biggest problem was that he was beginning to lose control of his bowels. He pooped on the carpet about every other day; to make matter worse, our carpet was cat-poop brown, so I'll leave you to guess how we most often found the evidence. He wasn't able to keep himself clean as he had in the past (sometimes I'd smell him coming up behind me), and he required frequent human-assisted baths, which were not a pleasure to him.
He probably could've lived another year, but his aging issues were compounded by the fact that we had two preschool children at the same time -- in a small house. When the first was born, he was able to deal with her, though he never really enjoyed her company -- she was a little rough on him. By the time the second came along, he just gave it up and spent most of the daylight hours in relative safety under the bed. Though we were working on teaching them to be kind to animals, they were toddlers, and they were often very rough in showing their love.
The final straw came one day when I awoke at 5:00 AM to hear my oldest crying, "Mommy, Kitty's in bed with me, and he's pooped all over us both!" It was true.
I just couldn't manage an aging cat who needed protection and the two preschoolers. Even though I knew he couldn't help himself, I couldn't allow him to poop on my children. So I decided to put him to sleep rather than see him wither away to even less than he was.
I loved that cat dearly, and it was a difficult decision. I cried. But it was the right decision. I had to choose the children's welfare over his, and he was released from his physical suffering, which -- being caused by aging -- wasn't going to get better.