Everyone who was either in the first or second beta has been automatically accepted back into the third. If you would like to verify anyway that you are in or to apply for a new account click where highlighted below.
When you've been accepted it will show this window:
Otherwise it will show this window (just check the checkbox and click the button and you're good):
After that you should be seeing the accepted window now.
I have tested it with an account under the age of 15, and you can't get in with one of those. I'm sorry for saying that you originally could. I assumed this based on the fact that it said anyone could get in and did not mention any age restriction at all.
They'll be allowing people to download the game client starting December 8th at 1 AM (EST).
Lastly, how it says the "cbt" in the url. They could just simply be using the same code to accept people into the beta and didn't bother to update its path. Like Nero said, by the definition of an open beta: this is an open beta.