They are actually exactly the same age.
Avoid the intervening stuff, and go to the last paragraph.
There was a schism around 2000 BC, when the combined tribe was still on the banks of the Oxus.
The winners believed Ahura was the almighty lord, and he was opposed by the Daivas.
The losers believed that the Devas were gods, and the Asura, their cousins, were the other divine band who were not gods.
The winners stayed on where they were.
First, the Mitanni broke away and migrated hundreds of kilometres and landed up finally in Asia Minor, today's Turkey, came to dominate the local people, the Hittites, and built a vast empire, one of the several that the Hittites were known for.
Second, the Indo-Aryans broke away, settled in Afghanistan, but not permanently stabilised there, called and named the seven rivers, the Hapta Hindu, until the bleak surroundings drove them across in dribs and drabs, as tribes and bands and even, perhaps, families, across the Khaibar, the Gumal and the Bolan. Those who came through the Bolan found a massive river in front that they named as they named other huge bodies of water, the Sindhu.
They found a people who had abandoned the magnificent Indus Valley Civilisation, and lived in smaller settlements outside those main cities, intermarried with them, and, in the northern branch, passed into today's NWFP, into the Punjab, and then through the Ganges Doab for nine centuries, until they reached the swampy mouths of the Ganges, by sometime before 600 BC. The western branch either started from a point where travellers would debouch from the Bolan (speculative) or were part of the original migrants who drifted downstream. Some time around 1500 to 1300 BC (difficult to date), nine of the southern kings met a late-coming tribe, the Bharatas, and were defeated.
The winners in all this schism won big, which is why Doc is so bloody insufferable. They penetrated into the Persian uplands, formed the Medean Empire, while smaller groups, speaking the same language, stayed back on the steppes and formed the Scythians. Ironically, the Medeans were supplanted by a vassal people of their own stock, the Persians, and their first great king, Cyrus, was killed battling his cousins, the Scythians, on the steppes.
Regarding religion, some time after the schism, a religious reformer united the Iranian speakers into the monotheistic Zoroastrian faith. Their religion was administered the way the Parsis and the Iranis of India today administer their fire temples.
The other branch, the Indo-Aryans, started with worship of the sky god, the god of thunder, the god of fire, the god of the waters, the goddess of the dawn, the heavenly twin horsemen, and in these respects almost exactly matched the beliefs of their long-lost cousins, the Mitanni. In addition, these peoples, having mingled thoroughly with the local remnants of the IVC and with the existing hunter-gatherers who populated almost the whole of north India, picked up the worship of local deities, and Siva probably dates from this intermingling, as do the goddesses, most of them who had not found mention in the Rg Veda. So you get the three main branches of what the people themselves never named, but today is increasingly being called Sanatan Dharma, the Old Faith, the three branches being worship of Siva, worship of Vishnu and worship of the Goddess. All that cultural churn happened between 1500 BC and 600 BC, when the Buddha and Mahavira threw out the tenets of the Old Religion, and declared that there must be a better way to live and to die.
