The age of an elephant is largely dependent on its teeth. Elephants have six sets during their life a new set replacing the one which has worn out. When there are no teeth left the animal will slowly starve to death.
There is evidence to suggest that elephants which have access to softer riverside vegetation will live longer than those who really have to chew their food as those on a rougher bark/tree diet.
The life span is roughly the same as a human...about seventy years.