You can get "fast-tracked" before it shuts to the public from now until "9:01am on Saturday, 7 May." Sign in to Games for Windows Marketplace for the beta.
"The team at Microsoft Game Studios wants to make sure everyone who wanted to get into the beta for “Age of Empires Online” gets their chance before they close it to the public, so they’re opening up access to get fast-tracked into the beta for 48 hours only this Thursday-Saturday," read the press release.
No waiting is promised, just sign in to the Games for Windows Marketplace and make sure you've got a Windows Live ID and then you're ready to download the beta. For more information about thevisit the official website.