Karen Stone, Intendantin at Theater Magdeburg, uses the criterion of one who commissions opera:
is the composer alive? can the composer respond to requests for changes?
I like this (binary) dividing line very much. A work that could potentially evolve, be revised, be re-scaled, a work that the composer(*) can comment on, is a work that has a different nature to one that is somehow 'fixed' in death.
The good news is that there are many living composers...
(*apologies to librettists. Your day will come)