From some posts on the Google+ discussion forum (https://groups.google.com/a/googleproductforums.com/forum/?hl=en#!topic/google-plus-discuss/RTh_laUTRFo https:/

From some posts on the Google+ discussion forum (https://groups.google.com/a/googleproductforums.com/forum/?hl=en#!topic/google-plus-discuss/RTh_laUTRFo https://groups.google.com/a/googleproductforums.com/forum/?h...): Google+ will likely have an 18+ requirement throughout the Limited Field Trial as they are still building the safety features for minors. Over the years there have a been a number of poor interactions between social networks and the laws regarding minors, and Google probably wants to try to head those off as much as possible. Probably due to US law, the same posts say Google+ will not be open at all for people under 13.