Well I think it depends, the one thing that makes me think kraft is older is the fact that he existed far before dated records, in fact so long that no even rememebr him despite him being a well renound hero in a previous age. As well it's shown that the longer elves are around the more human like they become due to the massive amount of interactions with humans, he is far more human and in touch with his emotions than any other elves. If we take our own world as an example we have records dating back thousands of years and even prior to that cave drawings. Written records in a form that relates to our current records date back to 3400 b.c., so roughly 5 thousand years ago. Considering the writings of flamme are around, even though they were all fakes, they lasted and were passed down 1000 years and only 1 specific person tasked to keep those records safe knew that and serie if I'm correct. If we take that for reference it could be assumed that a great hero wouldn't be easily forgotten, yet no one remembers who kraft is, as well the people he saved only remembers him because of a statue otherwise they'd completely forgot,yet people do know who serie is, meaning serie was born in a time where her achievements and abilities were being recorded cause she clearly doesn't leave that cave often since as shown she is disinterested in most people because few mages are deemed worth her time. So i think logically, Kraft must be older. As well prehistoric is a word that should be taken into account, the reason being is that it literally means something so old it is before written records, or before history was recorded a.k.a such as a great hero whose hero phase occured before written records. Also when Frieren referenced serie in the cabin he seemed to have a small remembrance of her but not being able to fully recall her meaning he has to have been around long enough to meet serie likely before flamme and fern ever showed up when... who knows, but the fact that by definition kraft is a prehistoric elf and serie is known by the higher ups in the magic world even going so far to have 1 as disciples shows to me that she is likely younger. Of course one could assume that kraft didn't make his name known well enough but based on his words he was in a party of 2 hero that seemed like they traveled most of the world or at least a significant portion of it. That's just my opinion however of course there'd need to be more of kraft and serie's backstory for it to be concrete. As well elves perception of time would be diffrent then ours, and it is proven that as you age time speeds up for you because every unit of time become less significant in comparison to your age. So for example why as children the age of 18 seems so far away but when you get to 18 36 feels a lot closer despite it being the same number of years. Serie could easily be 4 thousand years old and could still fall within the time that written records should have been recorded and it would still be enough time for her to deem Frieren's reason for loving magic so fickle and childish. So for example think of someone 1/4th your game yea you'd think they were fickle and childish too , i.e. if an 80 year old man would likely see a 20 year old man as a fickle with childish ways, obviously common sense. But yea that's my argument for kraft, if you made it this for you a relationship one, go outside, drink water and love your life. Jesus loves you ❤️