The real Merlin, Myrddin Wyllt, was born in about 540 and had a twin sister called Gwendydd. He served as a bard to Gwenddoleu ap Ceidio, a Brythonic or British king who ruled Arfderydd, a kingdom including parts of what are now Scotland and England in the area around Carlisle.

How was Merlin born?

Merlin's traditional biography casts him as a cambion, a being born of a mortal woman, sired by an incubus, from whom he inherits his supernatural powers and abilities, most commonly and notably prophecy and shapeshifting.

How old was Merlin when Arthur was born?

In Season one, Merlin is 16 years old. Arthur is 20-21. We know he turns 21 in Excalibur because it's his coming of age celebration.