Whales are marine mammals of the order Cetacea, and there are many species, but all whales have forelimbs modified as fins, a tail with horizontal flukes, and nasal openings or blowholes on top of the head.
Whales typically live between 77 and 211 years depending on the species, although the upper range has been questioned because the evidence of aging did not correlate well with other dating methods.
One fairly accurate way to tell how old a whale is, is to extract one of its teeth and cut it in half, since most marine mammals grow a new layer of tooth each year.
As with a cross-section of a tree, you can count the rings and know how many years the whale has been swimming the seas.
Unfortunately, since whales seem to have an aversion to dentistry, the hardest part is figuring out how to keep them in the dentist chair.