It really just depends on when it was built until it gets destroyed. We had 1 in my backyard until the last super storm (stupid) It had been there about 8 years, took a lot of hits, but they are fast workers & always repaired the damage asap. I don't think that they'll be rebuilding this time though, I have seen them nor any tracks or tree chewing
The age of the beaver dams mainly depends on when it was built and for how long it has been able to stay in contact. Some can be as old as 20 years, while others can be older than that, as long as the logs used to hold the waters are not destroyed.
There is a Beaver dam in northern Alberta that is almost 2800 feet long. It is believed to be over 25 years old. It is still in use and still under construction. There is another structure in The Yukon believed to be an ancient Beaver dam that may be over 10,000 yrs. old. But it is no longer in use. That one has Mammoth bones in it that are believed to have been brought there by the ancient beavers that built the dam. Of course the youngest beaver dams could be less than a year old and I don't know the average age of a beaver dam.