[Endoperez]

Crones aren't the only old creatures that survive only thanks to their magic. Bogarus has those cursed crone summons with age 110/200 or similar, the Crone pretender is always advised to take magic, etc etc.

IMO, having Nature magic bring Crones to the safe or almost-always-safe age category would be fine, so that if they are ever Muted or Feeble-minded, they'll probably catch a disease next winter.