Volterra.....

...is also a real place. Why do the Volturi look the way they do? Well according to Stephenie Meyer:

"For me it’s not like aging but it’s more like petrification, they never go anywhere, they’re not very active. In my mind it was like years and years of dust settling on their skin.
Tanya and her sisters in the Denali family are over a thousand years old but they look like Carlisle and they’re still very active."

(This is from a video interview with the Italian Twilighters Club)

And the about the Denalis:
According to Stephenie Meyer:

"Carlisle ran into the three main sisters earlier on, but they were just sort of looking into the vegetarian thing. They didn’t start out that way, for years and years they were the reason behind the myths about the succubus."

[In Western medieval legend, a succubus (plural succubi) is a demon who takes the form of a beautiful human female to seduce men (especially monks) in dreams - source: wikipedia]

"They really like human men, but those humans were meal after all. After awhile they started to develop a conscience, it didn’t seem right to have them as food, so they sort of evolved into vegetarian, whereas Carlisle was always that way."

(This is from a video interview with the Italian Twilighters Club.)