The Gathas were written about the fourteenth century B.C. by zoroaster and hence are not much later than the Rigvedas.
"The Literature of Ecstasy"
Albert Mordell
That this should put zoroaster at the time of Ninus, and make him king of the Bactrians, in order to allow him to be overthrown by the Assyrians, is very improbable.
"The History of Antiquity, Vol. II (of VI)"
Max Duncker
And as among the antients, there was not a proper uniformity observed in the appropriation of terms, we shall find more persons than one spoken of under the character of zoroaster: though there was one principal, to whom it more truly related.
"A New System; or, an Analysis of Antient Mythology. Volume II. (of VI.)"
Jacob Bryant