Greenland sharks have no otoliths and no annually banded fin spines, so conventional ageing methods fail. In 2016 a team applied radiocarbon dating to the nucleus of the eye lens, tissue laid down during embryonic development that is not remodelled afterwards, effectively a carbon timestamp from before birth.
Calibration used the bomb pulse, the spike in atmospheric carbon-14 from mid-twentieth-century nuclear testing, which marks lenses formed after about 1960. Applying that anchor across a size range of sharks gave an estimated lifespan of at least 272 years, with the largest individual placed near 392 years and a wide confidence interval spanning roughly 272 to 512 years.
Those intervals are the honest part of the result. The commonly repeated figure of a 400-year-old shark is a point estimate inside a broad range, and the associated estimate that females do not mature until around 150 years old carries similar uncertainty.