Up front, let me emphasize that no one knows for sure. Having said that, I visited recently, the university where I majored in Geology quite a few years ago, and had the distinct opportunity to discuss this issue with some graduate students and their lead professor. Latest studies actually focused on T-Rex since in the last few years, here in the western U.S., there have been a number of excellent fossil discoveries... in one case the Hell Creek formation in southeastern Montana and northeastern Wyoming provided the most complete example extent. Study of that and other examples indicates a probable life span of 30 to 40 years. There is speculation that some of the larger sauropods, like Apatosaurus, Brachiosaurus, et al may have lived to be 100 years old on average, discounting those suffering predation... Interesting question, as usual, warpig1...