"It is a judgement in the eye of the beholder ..."
Exactly. Unfortunately, the eye that matters is the YouTube staffer who reviews flagged content, and it is their decision that stands (though you can appeal and have a second review - did you try that?).
It MUST be decided on a case-by-case basis as there is no way to define with crystal clarity every possible scenario that anyone in the world might possibly record and upload. The PG-13 guideline is a good place to start, would you take a thirteen-year-old to a biknini wrestling match? Does that sound appropriate? Sounds creepy to me, but, that's my opinion.