The problem with dinosaurs is that they lived a very long time ago- between 250 million and 65 million years ago – and the vast majority of our knowledge of them is based on fossilized bones and skeletons, which are all that have survived the ravages of time.
To answer this question, we need to know two things about the remains of a dinosaur. Firstly, how old was it when it died? And secondly, did it die of old age?
Although we can sometimes tell from evidence of injury that a dinosaur met a violent end, it is generally impossible to answer either of these questions.
Estimates of the lifespan of dinosaurs are therefore based on what we know, from the evidence of present-day animals, of the connection between size and lifespan and the possible role of metabolism.
Calculations based on these factors have resulted in estimates from a respectable 75 years to an awesome 300 years for the lifespans of dinosaurs- a very broad range indeed.