Cat cats get senile (dementia) like humans?

My kitty, Smokes, is 16 years old. He used to be an inside/outside cat (along with his sister) and jump 10 feet from a wide concrete ledge outside the window in front to go out (I kept the window open enough for the kitties to go in and out as they please in nice weather) but in the last couple of years I have been letting him out through the front door to my apt. bldg., which is about 4-5 steps down as I'm pretty sure he has arthritis, and this was becoming too hard for him. If I got wrapped up in something and didn't let him in when he wanted to, he used to still climb/jump up onto the windowsill and bang on the window. As it got warm this spring, however, the two times he was out (I let him out as usual the first time, and the second time there was a lot going on and the doors were left open, so he just went.) My problem is that he stayed gone and wouldn't come when I called him in the back yard (he doesn't go into the street) and I searched for him for about 5-6 days, and decided maybe he was sick and "went away to die" like cats are known for. Almost ALL of my neighbors speak Spanish, and I don't, so it was hard to elicit help. I am disabled and can't walk far. Anyway, my neighbor upstairs on the 3rd floor that I'm friendly with had her cat get out, and in lookinf for him, found him with smokes about 4 backyards away, so I just rushed out and picked him up and brought him home and he was purring like crazy. He had lost weight as the only thing there is to eat out there is dry food my neighbor puts out for strays. So I decided not to let him go outside alone anymore, but he got out a second time, did the same thing (he can get shelter in the 3 garages) and I was frantic, but not sure he had died, like the first time. I thought I'd put a bowl of 9-lives canned food outside the window where he used to sit when he could still jump up, and when I went out, he was just sitting right there, so I snatched him up again.My question is, as I stated, can cats get dementia in old(er) age? It seems like he's happy to be outside, but is either deaf now or just ignoring me, and I can't go downstairs every 5 minutes to see if he's around. I probably should be vigilant about making sure he can't get out, but I'm wondering in changes in his behavior. The second time he was out, my ex husband (who he adores, I just am there to feed him and pet him when he wants when "Dad's" not around.) He likes, me okay, but when Lloyd saw him this last time on Saturday (he'd been out since Thursday) he ran away from him. Can anybody give me any ideas about what's happening with him? I've never had a cat act like this before, except when I was real little (like 3 or 4, we had a cat who had been in a cat fight and was older, and my mother explained that often cats "go away to die." I am on a fixed income and can only afford (barely and with the help of my ex husband) to take him to the Vet once a year to make sure his shots are up to date and a cursory exam - (It still costs about $175.00!) Thanks to anybody who can help me, I really appreciate it.