I believe the plan is to allow lots of warning tagging; the problem comes from both authors who don't like to use warnings, and from disagreements about what the warnings mean.
I think the current age concept for "chan" is about 16, but that might be flexible or changing based on legal issues. There's the potential weirdness that Jordangrant used in Cambiare Podentes: "in this AU fic, everyone is one year older than canon; Hogwarts runs from ages 12 to 18, so Harry is 18 when this begins." Which seems just... whacked. A nice dodge of the Australian issue, but I can't imagine that being considered acceptable as a general practice.