How old can a Greenland shark get?

Around 400 years, and the way that was established is as interesting as the number. The shark's vertebrae do not lay down countable bands, so age was guesswork until researchers turned to the lens of the eye - which forms before birth and is metabolically inert thereafter, so the proteins in its centre are as old as the animal.

Radiocarbon dating those lens cores gave ages up to roughly 400 years, with the largest individual estimated around 392 with wide error bars. The bomb pulse helped: nuclear testing in the 1950s and 60s spiked atmospheric carbon-14, and sharks whose lenses carry that signature were born after it, which anchors the whole calibration.

They do not reach sexual maturity until about 150. That makes them almost impossibly vulnerable to fishing pressure - a population cannot recover on any human timescale - and they were hunted for liver oil into the twentieth century. They swim extremely slowly, are often blind from parasites attached to their eyes, and are found with polar bear and reindeer remains in their stomachs, which nobody has satisfactorily explained.