To claim Zoroastrianism to be the oldest religion, you have to show that the religious orientation of the fist humans on earth was Zoroastrianism, or atheism which was later replaced by Zoroastrianism by many progeny of theirs.
On the ground of available history, one can only say: To determine the real oldest religion is not possible. For example, if your claim is correct, where is the historical evidence that before the origin of Zoroastrianism, humans did not exist on the face of the earth or practiced Atheism only? In absence of both, how is your claim established? Please explain.
The fact about our "thought process" is that even in the "age of reason", we go short-sighted in religion-related matters. We do not analyze a subject as much as it warrants, then fall into over-perception that we have done justice with our analysis.