Scholars agree that world's oldest living religion is the religion practiced by the majority of people in Bharata (India). It is pity that the oldest religion of the world is referred to as hinduism - a word that has been in existence for mere 1500 years. Its origin began in pre-history, over 3000 years BCE, when the would be civilized world in the land of Tigris and Eupharitis had not seen the light that eventually evolved in what is known as Judaism, Christianity and Islam.
Many scholars date the beginning of our dharma around 3300 years BCE. However the Vedic scriptures clearly indicate that this dharma has been in existence for infinite period - the chakra of this dharma has been turning for many yuga and for this reason it is called Sanatana - everlasting.
Judaism, Christianity and Islam - all have their founders. One person got enlightenment and his teachings became the basis of a religion. Sanatana dharma has no founder. This dharma is embodiment of the principles of righteous living - codes for the members of a society and spiritual thinking developed by sages over many yuga. These are contained in our scriptures - Veda. Upanishada, Purana , Shruti and others.
The scientific mind can go to 4500 years back when archeologists got the first knowledge of this religious practice from the excavations of Harappa and Mohenzodaro. Further excavations along the basin of Saraswati river has given more insight regarding the nature of dharma in pre - historic Bharata. Seals uncovered from these excavations give indication of religious practices involving female figurine depicting mother goddesses Shakti and Kali and lord Pashupati. Unfortunately the scripts of the seals recovered from all the sites along Sindhu-Saraswati have not ben deciphered as a result not enough information is available for the researchers to clearly comment about the religion at that time. However, the vastness of our scriptures, Veda, Upanishada and others and above all Shrimadbhagwat Gita ,all indicate a very high level of spiritual awakening that was prevalent at that time and that has prevailed as the oldest religion of the world through the ages and, therefore, must be referred as Sanatana Dharma and NOT Hinduism.