Question: How old is the oldest living mammal?

Thomas Butts answered on 14 Jan 2014:

That rather depends upon what you mean by ‘oldest’. If you mean, ‘which mammal has the largest life span?’ Then the answer is probably something very large like a whale.

If you mean ‘which mammal lineage is the oldest?’ Then the answer is all of them, because we are all derived from a single ancestral mammal species.

If, finally, you mean ‘which mammal species has been around for the longest as that species (it’s more ancient ancestors will have been a different species)?’ Then I am not sure it is possible to answer that, since our definition of what a species is depends upon their ability to interbreed successfully, and that of course is totally untestable with an animal from the past.