Dinos got really, really huge. What was their life span anyway, on average...say, for a full-grown T. rex? They must have lived a long time to grow as gigantic as they did.
-- TNT, the net
Of course, nobody knows absolutely, for sure, bet-the-rent, no arguments. Despite the fact that there is no real answer to your question, we'll waste this week's allotted space answering it. I love my job.
Dinos ranged from mouse size to house size, so life span was probably a variable thing. And based on skeletal evidence, dinos lived on the edge. Always getting into scraps with the neighbors, suffering drought and infections, dodging predators. Unlikely that many lived long enough to collect a pension. Dinosaur metabolism is another mystery, but it would have influenced growth-rate patterns and longevity. Most warm-blooded critters live fast, die young. Relatively speaking, anyway.
When paleontologists took their first swat at calculating dino life span, they estimated the mass of whatever species they were looking at and cranked that figure through equations based on known growth rates of similar animals. Age estimates ranged from maybe 100 to 250+ years. But since dinos are similar to both birds and crocodiles, for instance, these calculations were pretty much a crap shoot. As were guesses at the mass of a given dinosaur in the first place.
Recently we've studied dino bones kind of like you'd age a tree by its growth rings. So far the research suggests the animals grew rapidly at first, then stopped growing at some genetically determined point, more like warm-blooded animals than like crocs, which grow slowly and continuously. Estimates from these studies say some animals may have lived only a decade, while others took three, five, or more decades just to reach full size, and the hardiest might have lived more than a century. Of course, as some investigators point out, if calcium was absorbed out of the bones, that's another monkey wrench in the calculations.
As for your beloved T. rexes, few have been found, and they've all been roughly the same size. Without babies and juvies to study as well, there's not enough to go on. We may never know enough about any dinos to make reliable life span estimates. But at least you now have an idea of the kind of thing that fills up the hours in a scientist's life span.