I think the age is calculated counting the "rings" in ... [looking at google] ... the vertebrae.

They confirmed it with radiocarbon data, that is known to have their limits in modern samples since decades. Rings in the bones are known to be difficult to read and not very trustable. I know. I tried it. Is a nightmare. Unlike tree rings, this rings fuse, fork and vanish all the time. Different observers will come with different ages, to the point of the protocols for otoliths directly advise to take the average of three measures taken by three different people. You repeat the read one year after and obtain a different age for the same sample.

In cartilaginous fishes I bet that is much much worse. In young fishes the method can give a clue, but being able to count 200-400 clear rings is not realistic. At that point the error intervals are too big. We can't even be sure that we are not registering 200 meals instead.

Of course everybody just swallowed the glittering result as a proven fact, without much critical thinking, because it made for a great history. Saying "we don't know" wouldn't be publishable.

The only real way to prove this is to keep the animal in captivity from their birth so you are totally sure of his real age.