In one episode of Bewitched, we see a painting of Samantha (we assume it's Samantha) which is set in old Salem by the looks of it. This then raises the question of how old Samantha actually is, but the question is never truly addressed. So, how old is Samantha at the time of the series (educated guesses are acceptable).
There is a very extensive analysis of contextual clues from the episodes as to the various ages of the witches here
Ignoring the fact that several episodes specifically contradict each other, the general theme is that Endora is at least 3000 years old and that Samantha was (probably) born sometime between 1570 and 1600 which would make her approximately 390-400 years old at the time of filming.