personally I'm not a big fan of warnings, as an author I try to avoid them or keep them broad. I don't know if it's useful for readers at all; if I warn "this might squick some people" they complain that it wasn't squicky at all XD
generally to me "underage" means younger than 18, "chan" is younger than fourteen. that's how most stories and doujins seem to handle it (with the synonyms of shotacon/lolicon for chan)