What is that average age of a redwood tree?

It would be nearly impossible to compute an average, since there has been a lot of logging and there are now many 2nd and 3rd growth trees, so there is huge variation in the ages of trees, even in the same region. Many of them are about a thousand years old, some as old as two thousand years. I just visited Muir Woods this weekend, and they had a cross-section of a tree that had fallen. They estimated the birth of that tree around the time the magna carta was signed. One thing to note—newer trees grow thicker faster because they are 2nd growth threes that grew after areas were logged and there were no other trees to compete with, so two trees of the same girth are not necessarily the same age.

The only accurate way to find out would be to cut them all down, and log (no pun intended) the number and thickness of the rings, then calculate the average. I don’t suggest you carry this out though… A good rule of thumb is: if they’re tall and thick, they are pretty dang old. Here’s a good link, with a bit more helpful info:
http://wfi.worldforestrycenter.org/