How old did individual dinosaurs live to be? Like so much else about
dinosaurs, no one knows for sure. We can only look at related modern animals
and make educated guesses.
The larger modern reptiles, which are more or less distantly related to
dinosaurs, can live to be very old. Large crocodiles can reach ages of
100 or more; turtles can reach ages of 200 years or more. For these reasons,
it is reasonable, if not very likely, that large dinosaurs reached 100
or 200 years of age, perhaps even more.
If dinosaurs were warm-blooded, the comparison with reptiles is not as
valid. But when you consider that elephants and humans regularly reach
70 years or more, it isn't unreasonable to think that the dinosaurs --
and especially the larger sauropods -- lived to be at least as old.