I'm sure they keep living as long as you take care of them. And of course, they'll die if you neglect them.
Currently, my oldie is 34 years old and still going. She gets hungry quicker than teens/adults, and there was even one morning I didn't have school that I woke up late, and it had two piles of poopy on the screen and no hungry hearts whatsoever and a skull floating above her head after I cleaned the poopy, but she was still alive. Her eyes were blanking out, but after I fed her and took care of her, she was fine.
Or it could be that mine just doesn't want to die. ^_^