Properly, Sanatana Dharma cannot be called ‘Hindu’ or labled with any other such limiting term of reference, because BY DEFINITION it is eternal and universal. As the ordering laws of the universe or ‘nature’ cannot properly be called American, Russian or Indian (Indus = foreign word ‘Hindu’ for the people beyond the Indus River), Eternal Dharma cannot be said to be ‘Hindu’ or Indian, or time and place limited to any other collective of people or geographical region. If ‘Sanatana Dharma’ could be so limited, it would not be Sanatana Dharma. If you mean how old is some religion called ‘Hindu Dharma’, it would depend on exactly what ‘Hindu’ religion you are referring to. India / Bharata is the home of more than 700 languages and these peoples all have various religious views, which generally in modern times have fallen under the label of ‘Hindu’. I have traced-out the oldest of these living Traditions through over 40 years of exhaustive INTERDISCIPLINARY studies. From all of the available evidence of every kind, the Tradition that I have found with the greatest time depth and widest diffusion is the worship of HARI and/or HARA with Their Shaktis. The great sea and river basin civilizations all worshiped Hari by the Name of NR / MR / NHR / NHL or NRYN / NARAYANA. This is why the ancient Biblical Name of the entire Mesopotamian Region was NHRYM NAHARAIM / NARAYANA. In Egypt the Land of the NHL NILE was called MZRYM. NHRYM = MZRYM ‘Mizrayim’. Later MZRYM became NZRYN ‘NAZAREAN’. The NZRYN religion was that of the mikveoth or Sacred River Baptism practitioners. Thus in India, we can still see this practice in the Kumba Mela and in the other ritual ablutions performed in Sacred Rivers like the Ganga and Jamuna. Such purifying rites are also performed in kunds, lakes and ghats.
In Rome and Europe, the Supreme Lord (HERU / HARI / SOLE / HELI / ELI etc ) as NILUS or the Lord of MZRYM (Egypt the Land of the NHL / NILE) was called Zeus-Jupiter MARIMUS. MARIMUS = NARAYANA. Thus from Egypt to India, the first Agriculture-based Sacred River-course High Civilizations were ALL founded on the worship of NARAYANA and His Shakti, NARAYANI.
In Rome Jupiter Marimus was worshiped with Juno Marina. These water-related Traditions of HARI as NARAYANA were PRE-HISTORIC, so what can we say about their time of origin? In each region we can get dates from the geological record, from specific sites of ruins and from artifacts, but we cannot really tell when or where the phenomenon of Narayana and Narayani Worship began.
At Rome Marimus was worshiped as the Lord of the Great Tiber River. The Nazarean Sect of the Jews performed their Baptism Rites / Rityas in the Sacred Jordan ‘Descender’ River. The Jews and Egyptians believed that their Sacred Rivers descended from the Foot of the Lord HERU-AUSU / ELI-YAHU (as NHRYN / MZRYM) seated on His Throne in Heaven! This teaching is preserved in the cCatholic Apostolic Christian New Testament of the Bible, where the Jordan is described as flowing from the Foot of the Lord on His Throne in Heaven.
In the Bible, Jesus is recognized as that Same Lord, and His Baptism in the River Jordan by Saint John the Baptist, the Nazarean Leader of the time, recalls the Vaishnava and Shaivite stories of the Heavenly Ganga pouring through Siva’s ascetic’s (Nazarite’s) unshorn hair-locks AND the Siva Lila of periodically entering the Ganga to take-away the sins in it! Thus when Saint John the Baptiser saw Jesus, he recognized Jesus Himself as the Lord of Baptism / Ablution ‘Who takes away the sins of the World’ and thus John said that he needed to be Baptised / Purified of sin and SAVED by Jesus!