Not much is known about Merlin’s origins. It’s speculated that he has parentage of both mortal and supernatural descent. He seems to age at a decelerated rate as he doesn’t look remotely different from the time of Uther’s deception, to Arthur pulling Excalibur out of the stone. Which would enforce the supernatural parentage theory.
The only known records of Merlin are documented at the time that he started being an advisor to King Uther. After a time he aided in the King’s deception and conception of the heir known as Arthur. Merlin took Arthur at time of birth to be raised by Sir Ector, concealing the infants parentage to give him a childhood that would define his character as a man and later a King, rather than being a spoilt prince. Merlin watched over Arthur as he grew, teaching him the wisdom that he would need for later kingship.