Any time a pet doesn't act his or her self, there may be a medical cause. There are a ton of metabolic problems that can lead to a glassy-eyed confused cat. Renal failure, hyperthyroidism, liver disease all come to mind. I'd get the cat checked out by a vet with bloodwork ASAP because something serious could be going on. 13 is only middle aged for a cat, so something else may be going on.

Yes, see a vet! My cat had a stroke.

Yes, cats can get a little bit senile - normal part of aging, but what you are describing may be physiological problem. If your cat is stopping mid step and staring and is definitely not "all there" he/she could be having a seizure. Go see a vet right away.