As far as I know, there are two categories of YA. Ages 12+, and Ages 14+. I believe the early Harry Potter books were sold as MG.
Okay, found it. Agent Kristin Nelson (represents Sarah Rees Brennan, Allie Carter, Gail Carriger, etc) talks about it here: http://pubrants.blogspot.com/2012/02/fridays-with-agent-kristin-episode-2.html
Her main point is the difference between MG and YA, but she defines each category before going on to say what differentiates them.
New Adult aims for a slightly higher age range, 18-30. http://sjaejones.com/blog/2009/new-adult-shelving/ That’s an old blog post, back when people first started talking about New Adult as a separate category, but the writer of the blog acquires books for St. Martin’s Press…