So...
There have been plenty of threads where we have tried to work out Serana's age. We have it accurately pinned down somewhere between the Merethic Era and the Second Era. Her speech tends to be contradictory; suprise at the Dwemer being missing, and Cyrodiil being the seat of an Empire, yet at the same time knowing the land of Cyrodiil and recognising Skyrim landmarks that wouldn't have been there. Further muddying things is that her tomb is either built inside/beyond an ancient Nord barrow, or a barrow was built around her tomb, and other small details.
I was hoping we could try and put together a coherent (if vague) background from what she says as you travel with her and what we see in her crypt and Castle Volkihar.
I, however, haven't played Dawnguard recently or taken Serana to many locations. So while I go read up, would someone please do the honor of starting with more solid info?