Apparently Google+ has never officially been for those under 18 until now -- although no one's likely been patrolling for it. The standard age for Internet use in the United States is 13, set by the federal Children's Online Privacy Protection Act, and is the age limit for most social networks including Facebook -- although many younger children use it even with the permission of parents.
Horowitz's post mainly pointed out all of Google+'s features, such as Hangouts (just what you want your 13-year-old on), sharing content and connecting with friends and family, but seemed written for a much younger child and had a distinctly odd vibe to it.
Our newly launched Google+ Safety Center describes these and other changes in more detail, but our approach is straightforward: build awesome features that teens really want, encourage safe behavior through appropriate defaults and in-product help, and make abuse reporting tools easy to find and use. . . . The joy of real-life sharing lies in connecting with everything we care about—from family and friends, to businesses and brands. Teens, like adults, deserve a rich experience online, so today we’re welcoming some of their favorite individuals and organizations to the Google+ community.