The Italian Website for SerieTiVu gives us an article which claims the confirmation of season 6. However no source has been mentioned which leaves the credibility of the article in question.
I have translated the article for the non-Italian people below.
Finally, we can say with certainty: Merlin does not end with the fifth season, but was renewed by the television network BBC to come back with the story of the young Merlin for next year.
The news was not sure and there was a series of negotiations and rumors going on, especially because many indicated the feature that will soon be realized as final for the series, but now there is an official confirmation that the sixth season of Merlin will be there.
Filming will start at the end of the year in the basement of the castle of Pierrefonds , a destination that has already attracted the crew to begin shooting and some die-hard fans: what do you think we can expect from this sixth season of Merlin ?