User blog comment:Aithusa07/Merlin's Birthday???????/@comment-73.175.53.229-20160502202609/@comment-5674726-20160502231009

They don't share a birth year.

The Great Dragon was captured after Arthur was born, and it was only after that that Balinor fled to Ealdor and impregnated Hunith. Arthur turns 21 in the latter half of Season One, which would make him about 20 and a half at the start of the series. Merlin arrives in Camelot on the 20th anniversary of the Great Dragon's capture so the oldest he can be is 19 and a quarter.