For those who are interested in the period aspects of the vampires in Twilight you might want to checkout 'Twilight and History' by Nancy Reagin.
I stumbled across it when I was searching for the name of Maria's sire and what little I read looked interesting if only for the fac that there's a big old chunk devoted to our favourite Major. I make no claims whatever about the accuracy of any of the content. I do intend to read the chapter about 'Jasper Hale; The Oldest Living Confederate Veteran' though, if only to see if they can shed any light on the 'age & date of change' connundrum.
It's available for partial reading on Google books online. I will be checking to see if it's in my local library.
Incidentally, the 'look inside' versions on Amazon have a timeline comparison of human -vs- Twilight history from the creation of the Volturi onwards.