I guess it could be some kind of time magic effect, but probably not because of delayed aging. Maybe it's just a little known side-effect of extensive time fuckery that isn't normally an issue because it's such a rare occurrence? It might not be related to time magic at all, but instead a result of sacrificing other Valkyries for their powers.
Unless she lost her Valkyrie-based agelessness when she became Forsaken? That would make sense if some aspect of a Valkyrie's power is continually granted by Odin, or must be renewed regularly. Her forays into time magic and necromancy could have been motivated, at least in part, toward maintaining her youth.
I doubt she lost her agelessness on betraying Odin.
Thats a vulnerability in his followers I doubt he'd tolerate, because someone hostile would figure out how to spoof it at the worst possible time. And those types of strings breed resentment.
But it could be a consequence of sacrificing other Valks for power, or backlash from excessive use of time magic.
Or even from her use of necromancy.
Between Molly and Bob they could probably come up with a couple theories.
@DragonParadox
FURTHER COMMENTARY
Dresden's report to the White Council on this event is going to make for fascinating reading.
For one thing, he knows there's Black Council infiltration, so his immediate inclination is going to be to limit what he says in an attempt to protect Molly. On the other hand, Molly went full incarnate archdevil in this fight before multiple witnesses, and I would be very surprised if people couldnt feel us going shintai from across the city, so he has to tell them something.
Not to mention decide what to say about EvilBob who is out there somewhere.
I do wonder if Kattrin's effects have enough information to point at other members of the Thule Society, and provide independent corroboration of Molly's naming of the C-suite.
The WC is going to have varying shades of alarm and relief.
Alarm that female mini-Kemmler survived the Darkhallow even after being shot in the presence of Wardens; anyone who was involved in hunting Big K down probably would prefer to avoid a repeat. Relief that a kill was confirmed this time by three independent witnesses, all agents of different Powers. Paranoia that she will pop up again.
I would not be surprised if Harry has to meet Luccio in the next week for a face to face.