In the twilight saga Carlisle was born in London 1640 so doesn't that mean he's British? Shouldn't he have an English accent then?

I would imagine that the character would have a slight English accent, but since the character originally appears in a written work, where voices obviously aren't heard, a reader wouldn't make that connection, and an author wouldn't dwell on it much. Since there was no obvious mention made of any accent made in the books, the movies wouldn't have included one either. You could also consider the fact that centuries have gone by, and it's made clear in the books that Carlisle has resided in several different places over this time, and has lived in America since at least the 1920s (when he met Esme), so one could assume with confidence that he lost his accent almost entirely.

After living for almost 400 years in a NON English society? No. My grandparents and there siblings came from an area with a strong English accent, very thick. About 10 of their group emigrated to Canada After 20 years, those who had maintained ties to their English heritage (joining clubs etc) kept their accents, in some cases too strong to even understand. Those who joined the mainstream lost them completely.

I'd imagine after a few hundred years of hanging around with Americans, he lost the accent.

I think he lost the accent over the hundreds of years here in America.

if you were born in England but grow-up in another place you would not have an English accent

also you can lose your accent just from being in another country long enough

in the book, they have had so much time to learn multiple languages that he assimilated into an american accent, in order to fit in :)

People lose their accents.

It is all fiction.