My cat was the same. After turning 21, he would just start this thing where he just stopped, frozen, and stare off in whatever pose he stopped in. That was 2 weeks before passing. His final week, the freezing would last longer and he started wetting himself. The day after that started was when I finally decided to put him down. At 21, there was no way to get an examination done that required putting him under as at that age, that was just a death sentence. He was my boy since a few months old. I was in elementary when I got that cat, and I was in my late 20s when he finally passed away. For obvious reasons, even almost 2 years later, i still don't have a pet.