Bo was 14. This last week he didn't have much of an appetite. Then Wednesday he started losing his balance a bit and getting weaker, and his breathing was very labored.
We took him in to the vet, who said he might be developing an ulcer, and might have cancer, because we found a couple growths on him. So we brought him home with some painkillers and something to soothe his stomach.
Bo's breathing got more labored all day, and he didn't want to eat or drink at all. About 9 pm, I gave him some water in a syringe and he perked up a bit, went into the kitchen, laid down on a rug and dozed off. A few minutes later I went to check him and see if he wanted more to drink, but he wasn't breathing anymore.

Seems to have been painless. Not real sure why it happened, his bloodwork at the vet came back normal.