He first appeared in literature written by Geoffrey of Monmouth c1133. Geoffrey is believed to have been cribbing from much older now lost works, some that might have actually have been penned by the Bard himself six centuries earlier.
Rumor and hints of this person keep popping up throughout history. The literary references cannot be trusted at all. Merlin has become an item of popular culture. Hints from historical sources indicate this man, or someone claiming to be him and having sufficient knowledge to back the claim keeps popping up. The Elizabethan Era magician Dr. John Dee has a creditable claim to the title of Merlin. A little known letter from Thomas Paine claims that Benjamin Franklin was Merlin. (Paine was known to be a little flaky in his later years and the claim must be regarded as suspect.) Everyone from the Venerable Bede, to Isaac Newton, and Winston Churchill has been suspected of being Merlin. Among certain sources if you were smart, innovative, and well known, you must have been Merlin.
Is he still alive and working on his own agenda? If so he has become excellent at hiding his presence. We hope that if he does exist and is of a goodly nature that the increasingly public role of magic will draw him out.