Scientists do not know the exact lifespan of the dinosaurs, but they estimate that dinosaurs lived about 75 to maybe as much as 300 years. This educated guess is based on examining the microstructure of dinosaur bones, which indicate that the dinosaurs matured slowly. This is similar to ancestors of the dinosaur, including crocodiles, whose eggs take about 90 days to hatch, and whose lifespans can extend from 70 to 100 years.