We had a long discussion among a lot of AP staff with consults from their children of all ages. In our stories, we went with "6-7," because it's how kids say it: They don't say 67. Even though six-seven written out would technically our style, it was considered the least favorable option, as it goes against the casual way the phrase is used. We did not, however, manage to come up with a definition. My own kids just laughed when I asked them. I've also noticed that now that adults have started saying it, it has quickly gone out of style. 

We don't have an entry on outback. Merriam-Webster doesn't capitalize outback, so we would follow that guidance, although I see your point about how it might be similar to the South. I'll add it to our list to look at. Thanks! 

outback  noun : isolated rural country especially of Australia
Collegiate  /  Chapter O

outbacker  noun : a native or resident of the outback
Unabridged  /  Chapter O

the outback  noun : the part of Australia that is far from cities and where few people live