We just had Boo-Boo put to sleep this past spring, and he was almost 16. I had had him for almost as long as I could remember. It was one of the hardest things I've ever had to do.
It was hard on Cujo, too. He didn't speak for two weeks after that.
Also, Boo-Boo was in great health, and only really started to go downhill the last year or so. He really had 15 years of perfect health.