Psst: For the people finding her "at least a millennium" line to be some kind of a big reveal about her age, refer back to chapter seven-- where she openly (and rather flatly) comments on how it's been over a thousand years and her thoroughly deceased mortal master is STILL able to manipulate her.
I'm more intrigued about how young some of the "strong" demons they're facing are, considering both elves and demons are ageless (on a human scale, at least) and they be been in conflict at least since Frieren was young...er.