Part of the Google Blog announcement from today:
http://googleblog.blogspot.com/2011/09/google-92-93-94-95-96-97-98-99-100.html
+Everyone

100. Anyone can sign up for Google+—no invitation required.
For the past 12 weeks we’ve been in field trial, and during that time we’ve listened and learned a great deal. We’re nowhere near done, but with the improvements we’ve made so far we’re ready to move from field trial to beta, and introduce our 100th feature: open signups. This way anyone can visit google.com/+, join the project and connect with the people they care about.

Over the next day we'll be rolling out all of these features globally. In the meantime, you can check out what's next in Google+.


Unfortunately, they neglected to clarify that those under the age of 18 are still not able to join until Google has their teen safety features in place.

Users who want the latest information on the age issues as they relate to Google+ may want to move on over to the Google+ Forum thread:

Age and Google+
https://groups.google.com/a/googleproductforums.com/d/topic/google-plus-discuss/xVP6We0N1N0/discussion