According a biographer of Sri Ramakrishna (Vaikunthanath Sannyal : Sri Sri Ramakrishna Leelamrita), in the Vedic age, there were no FORMAL sannyasa. It was started by Sri Adi Sankaracharya 'most probably' by following the 'Tantrika-Paasupata' system.
Is there any evidence of whether its just an assumption?