My late chow-Lab mix, Ruth, lived to 16 but started Doggie Dementia at 15.
At first she would just space out. We called that "forgetting her keys" or "staring at invisible gophers."
After about six months, she started peeing in the house occasionally. Not as funny, but still tolerable.
After another six months, she started snapping randomly at me. Not little nips, full-on snarl-bites. I had her put down at that stage.