[Gengis Khan]

The oldest of the "Aryan" language is Sanskrit (not Farsi or any other European Language) which was/is spoken in India. It was spoken at a time when many of the languages like Farsi, Latin, Celtic, Greek were not even born. Most of the Indo-European Languages derive their grammar from Sanskrit. Sanskrit is also the language in which the Vedas (including the Rig Veda) were written.

[Verginius Rufus]

The other Indo-European languages, outside of India, did not derive their grammar from Sanskrit--they are sister languages, not daughter languages. It would be like saying Portuguese and French derive their numbers from Romanian. The words are indeed similar in all three languages, but it's because they all derive from Latin. Sanskrit is already many centuries removed from the parent language, "Proto-Indo-European," which was probably never written (at least no texts in it have been found).