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?

She was sealed away somewhere between the building of Winterhold (1E???) and the establishment of the first Empire (1E243). Other than that there's not much to go on.

Relevant bits of dialogue:

• "Nordic ruins. Even older than I am. I wonder if the draugr are as gullible as they were when I was a girl."
• "Is this a dwarven city? I can't believe they'd let it get so run down."
• "I always wondered what the dwarves actually looked like. I hear they're like elves, but with beards."
• "I'd read stories about the Solitude windmill, but I didn't expect it to be that big!
• "From the castle, you used to just be able to see Solitude over the mountains. It's exactly what I imagined."
• "They used to call Windhelm, "the City of Kings." In my books anyway."
• "Cyrodiil is the seat of an empire? I must have been gone longer than I thought. Definitely longer than we planned."

I'd say the two bits about the dwarves seem a bit contradictory - she shouldn't be surprised that a city got run down when all the citizens disappeared, but at the same time she uses the past tense when talking about the Dwemer themselves - which would suggest she knows about their disappearance. This also fits the fact that she knows about Solitude and the windmill there (though I'm not sure how old it could be, I doubt it is such an ancient relict). The last quote is the reason most people say she's from the first era - but I'd say that's reading a bit too much into it (for all we know she could live during an interregnum planning to go to sleep for a few decades - in such a case having a new Empire would be surprising).

Well, even if she was there when the Dwemer still existed (so she gets surprised to see a Dwemer Ruin) she still could have never seen one herself (So she now wonders how they looked like)