Crocodiles can live up to 100 years, but they do not die of old age!
After a certain age, they simply stop feeding and die of starvation.
edit:
It's true that I forgot to explain how that's possible. So here's the clarification:
Crocodiles are constantly growing; the older they get, the bigger they are, but the bigger they get, the more they need to eat.
Thus, there comes a point where they simply cannot hunt enough food anymore.
But does that mean that, theoretically, a crocodile in a zoo with a good diet would be immortal?
Yes, but that's only in theory. When a crocodile gets very, very old (and therefore very, very large), its legs don't have enough strength to lift its body (which weighs over a ton). And when the crocodile can no longer move, it refuses to eat, even if you put food directly in its mouth, and will simply starve to death.