Mehrdad, unlike the other respondents, I will disappoint you with a meta-answer. But it is the truer answer.
There’s no such thing as an older language.
Let me transpose the question to Iberia. People often say, “Woah, man, Basque is like, the oldest language in Europe, man! It’s like, as old as the Cro-Magnon!”
That’s crap. Basque is as old as Spanish is.
“But 2500 years ago, they didn’t even speak Spanish, man! They spoke, like, Latin!”
Yes. And 2500 years ago, they didn’t speak Basque either. They spoke Aquitanian language. Which was the ancestor of Basque.
“But man, they didn’t speak Latin in Iberia, man! They spoke that in Italy! And they totally spoke Aqui… Accu… Aquaman in Iberia, man!”
Yes, but you didn’t say “what is the longest continuously spoken language family in Iberia, without involving major territorial changes.” You said “oldest language”. And Basque is no older than Spanish in that regard. There was an ancestor of Spanish spoken 5000 years ago (somewhere far to the east), and there was an ancestor of Basque spoken 5000 years ago (maybe closer to Spain, maybe not).
So. Same for Farsi and Arabic.
- In 600 BC, we have in inscriptions in Old Persian, which is an ancestor of Farsi. (But is not, itself, Farsi.)
- Much later, in 150 BC, we have Pahlavi texts in Middle Persian, which is a closer ancestor of Farsi. (But it is still not, itself, Farsi.)
- At around 150 BC, we have the first indications of Arabic, as names embedded in texts in the Nabataean language. So we know that Old Arabic was spoken in 150 BC. (But Old Arabic is not, itself, Quranic or Modern Arabic)
So, OK, Old Persian is older than Old Arabic. But Arabic did not drop out of the skies into Nabataean. At around 600 BC, people were speaking an ancestor of Arabic. Something *like* Dadanitic (though probably not Dadanitic itself).
So you tell me:
- 600 BC: Old Persian — Dadanitic
- 150 BC: Middle Persian — Old Arabic
- 800 AD: Early Modern Persian — Classical Arabic
- 2000 AD: Farsi — Modern Arabic
Is one older than the other? Why? Because it has the word “Persian” in it? Because it might (*might*, I actually don’t know) have changed slower? But that doesn’t make it older. Old Persian is still not the same as Farsi, any more than Dadanitic is completely different from Modern Arabic.
Do we have more old written literary texts in the Persian branch than in the Central Semitic branch? Sure. But that’s not what “older” means. That’s what “older literary use” would mean.
And FWIW, Dadanitic was spoken in traditional Arabic territory, just like Old Persian was spoken in traditional Farsi territory. So we don’t even have a scenario like Basque vs Spanish, with Latin coming into a new territory.