How old do you think Samanta was? According to how the Witches seem to age, I'm guessing around 500 years old?

Well, it was established that she was alive at the time of Shakespeare and Henry VIII. Probably 600 years old or so.

It seems Genies age better than witches lol

Serena said Endora admitted to being 1000 years old, SOOO I'm guessing she MIGHT be arund 1200 or 1300 years old, but Jeannier was over 2000 years old and looked young. Jeannie would have been way older than Endora.

So, was there a backstory of why a 600 year old entity with supernatural powers suddenly decide to be a mortal in 1960's suburbia? To Darren Stevens of all people, a somewhat clumsy advertising guy working for a third rate firm under a clown like Larry Tate?

Actually the claim was made early on that McMann & Tate is one of the nation's top advertising firms -- though one wonders why there seems to be a real scarcity of employees apart from Darrin, Larry, and the secretaries...! lol

Not to mention why Darrin always seemed to be worried about money and telling Sam to cut back on expenses...but hey, it's a sitcom and logic is always the first thing to fly out the window.

there was an early episode where she said she's something like 600 years old.

That to me was a plot hole, but remember Samantha eschewed using her powers unless they were a last resort. Also, Samantha could not undo spells of others, which made them dangerous, even the "nice" ones like Dr. Bombay, Aunt Clara and Uncle Arthur.

She had to limit her powers to find a mortal mate because if she did not, she would not know if the mortal loved her for her or just to have carte blanche on her powers.

For a silly sitcom, there were some sci-fi elements to it.