Ageing an urchin is indirect. Researchers have used growth lines laid down in the calcite plates of the test and in the jaw elements, and have cross-checked these against tags and against radiocarbon signals from mid-twentieth-century atmospheric testing. Those approaches converge on lifespans for large red sea urchins measured in many decades, with the upper published estimates exceeding one hundred years.
What drew attention was not longevity alone but the apparent absence of expected senescence. Old individuals in sampled populations continued to grow slowly and to produce gametes, and mortality did not appear to rise sharply with size in the way it does for most animals.
The caution is real. None of these are birth-to-death observations of a single animal, and growth-line methods carry assumptions about deposition rate that can be violated when food supply varies. The widely repeated figure of two hundred years should be read as the far end of a modelled estimate, not a measured maximum.