I've been thinking about how old Serana, and Harkon, are. However as i've only watched a lets play of the Dawnguard side I haven't got to converse with Harkon a great deal and wll be focusing on Serana.
Serana when she wakes seems not to knwo the Empire. Furthermore, when you state the Cyrodillic Empire, she seems stunned that Cyrodiil is the seat an an Empire. This... is where it gets tough. With a couple of gaps there has been an Empire since the Merethic Era. A Cyrodillic Empire at that. She doesn't make it sound like she is suprised there is one now, she is suprised there is one at all.
Am I just misunderstanding her? Perhaps i'm wrong, and the Empire was in one of ts 'collapsed' states she she was simply suprised it was back. I find this very unlikely though. I still feel she was suprised there was an Empire in Cyrodiil at all.
That would put her at older or as old as figures like Vivec, Almalexia, Sotha Sil, Dagoth Ur and even Divayth Fyr.
A little mini-timeline I made to try and find somewhere she could have 'lived'.
1E 1
1E 139
King Harald orders a campaign against the remaining Snow Elves in the region near Lake Honrich.
One of the last remnants of the Dragon Cult is discovered in the secluded monastery of Forelhost.
1E 2920 - Morag Tong assassinate King Reman Cyrodiil III.
There was then a unknown period between the 1st Era and the second Empire, when the 2nd Era began.
2E 533 - The events of TESO take place, and its very relevent. Molag Bal is highly invovled and a great number of people have their souls taken (Sound familiar *cough*Harkon*cough*). I suspect this is actually around the time things happen, but its doesn't quite fit. There has been an Empire in Cyrodiil a short time before, and the throne is going to be fought over.
2E 897 - Tiber Septim declares an end to the second era, and thus, the Third Era begins.
3E 433 - The Oblivion Crisis takes place, and Emperor Martin Septim declares the beginning of the Fourth Era.
4E 201 - Events of Skyrim, and Dawnguard.
So. If she is became what she is and years later she was sealed away around the time of TESO, she would be more than 998 years old. Unlkely because of her suprise. If she was born in the unknown period between the First and Second Era, then She would be over 1500 years old. However, I still thin she was born in the Merethic Era, which puts her at over 4000 years old.
I just find it interesting how Serana carries herself. She has none of the gravity of the Tribunal, and noen of the world weary wisdom of Divayth Fyr. Then again, I suppose Harkon does seem every inch the ancient Vampire, and she has been locked away potentially most of that 'whichever' age. I'm still going with 4000. When we release her our hand is spiked and bleeds; I assume the blood funnelled down to her, and woke her from a sleep. Or Torpor as other vampire sources would put it. So while ancient, mentally she is just 'young' (we ahve no idea the space between her becoming a Vampire and her sealing) and out of touch.
So. Thoughts? It seems a pretty interesting thing to discuss, though perhaps better suited to copy-pasta in the Lore forum when Danwguard is more widely released.
More things that I might have missed on the Lets Play would would be interested. Things said by the family and other Vampires. Things we can sue as time markers.