Which is oldest known sacred place of Hindus?
I have found several temples & sacred places of the Hindu religion to be told as many thousand years old. Which of them is oldest known such temple or sacred place?
The oldest sacred place in Hinduism is probably Kurukshetra. Most people today know it as the location of the Mahabharata war, but long before then, it was the site of a Yagna (fire-ritual) performed by the gods. Here is how it's described in the Shatapatha Brahmana of the Yajur Veda:
The gods Agni, Indra, Soma, Makha, Vishnu, and the Visve Devâh, except the two Asvins, performed a sacrificial session. Their place of divine worship was Kurukshetra.. Therefore people say that Kurukshetra is the gods’ place of divine worship: hence wherever in Kurukshetra one settles there one thinks, 'This is a place for divine worship;' for it was the gods’ place of divine worship.... They spake, 'Whoever of us, through austerity, fervour, faith, sacrifice, and oblations, shall first compass the end of the sacrifice, he shall be the most excellent of us, and shall then be in common to us all.' 'So be it,' they said. Vishnu first attained it, and he became the most excellent of the gods; whence people say, 'Vishnu is the most excellent of the gods.'
So long before later texts like the Ramayana, Mahabharata, and Puranas described a wide variety of sacred places, we have a declaration from the sage Yajnavalkya (the sage of the Shukla Yajur Veda) that Kurukshetra is "a place of divine worship".
And Kurukshetra's holiness was recognized throughout the ages; as I discuss in this answer, Krishna was reunited with his foster parents Nandagopa and Yashoda because people from all over India made a pilgrimage to Kurukshetra during a solar eclipse.
As per Rig Veda, in order to settle the dispute between Lord Bramha and Lord Vishnu, Lord Shiva is said to have manifested as a column of light, and then the form of Arunachala. Arunachala hills in Tamil Nadu is said to be that place. Reference can be found here.
As per Rig Veda and Shiva Purana, the oldest inhabited place known would be Kashi (ie, Banaras or Varanasi) which was build by Shiva and Shakti, when they create Purush and Prakriti.
The following extract from this wikipedia article also throws more light on this from the archaeological point of view:
The earliest known archaeological evidence suggests that settlement around Varanasi in the Ganga valley (the seat of Vedic religion and philosophy) began in the 11th or 12th century BC, placing it among the world's oldest continually inhabited cities. These archaeological remains suggest that the Varanasi area was populated by Vedic people. However, the Atharvaveda (the oldest known text referencing the city), which dates to approximately the same period, suggests that the area was populated by indigenous tribes. It is possible that archaeological evidence of these previous inhabitants has yet to be discovered. Recent excavations at Aktha and Ramnagar, two sites very near to Varanasi, show them to be from 1800 BC, suggesting Varanasi started to be inhabited by that time too.
Most evidence of temples show that they arose after the Buddhistic era. There have been found the ruins of public gathering places that were probably used for worship purposes at Mohenjo-Daro dating from about the 3rd millennium B.C. (Myths and Symbols in Indian Art and Civilization by Heinrich Zimmer)
The earliest place of worship was the cave of the heart. The upanishads reference that the Atman dwells in the cave of the heart and is about the size of a thumb.
When Lakshmidhara wrote his compendium of tirthas in the twelfth century, he seemed to emphasize, at the outset, that real tirthas are not the places alone, or the sacred waters alone. The real tirthas are truth, charity, patience, self-control, celibacy, and wisdom--these are the tirthas of the heart on which one must bathe. He quotes from the Brahma Purana: If merely going to a tirtha is enough to purify, then the fish of the Ganga and the birds who roost in the temple towers would be instantly purified.
(India: A Sacred Geography by Diane L. Eck)
Rameshwaram is one of the oldest and a holy place in India. It dates back to the Hindu epic of Ramayana which took place in 15000BC.
Rameshwaram, as the name suggest was the place where Lord Rama prayed to God Shiva (Ishwar). Unsurprisingly, there are two different tales that revolves around this event.
One legend mentions that Ram prayed to God Shiva before his army built the Ram Sethu Bridge to reach Lanka to save Sita and defeat Ravana. Another tale mentions that, once Rama killed Ravana who is a brahmin, he prayed to Shiva to wash away his sins for killing a brahmin.
Rameshwaram, till date is considered as one of the most sacred place among Hindus.