Absolutely not "banned". Movie theaters customarily won't let children under 17 into R-rated films without a parent, but there's no law that I'm aware of enforcing that - it's purely an industry custom. And there's certainly no analogous restriction when purchasing an R-rated DVD, or watching one on a streaming service.
Oh wild. Learn something new every day. I worked at a movie theater myself and totally thought it was a law that people under 18 had to be accompanied by a guardian by law.
The very reason this isn't legally enforced is because industry self-regulated this way (to avoid legislation that would come with actual liability). If the Internet made any effort at self-regulation beyond "say, you're over 18, right?" perhaps we wouldn't be in this situation.
