she was being kept alive by the lord of light's magic. she was actually like 400 years old, and used magic to appear young. we'd previously seen her in her 'actual' form when she didn't have the necklace on a few seasons back; when she took her necklace off after winterfell, the light went out from the stone in the centre, signifying she was no longer under the lord of light's protection. kinda like beric; she was being kept alive to serve a purpose, and when that purpose was served, she was allowed to die