User blog comment:Emily Windsnap/How old are the Merlin characters?/@comment-93.97.109.41-20140119001550/@comment-5102537-20140119120249

Arthur turned twenty-one in "Excalibur" in season one when he became crowned Prince. Him telling Merlin in "The Wicked Day" that his father fought magic for twenty years was an error in the timeline, a simple plothole.

In episode one of season one, "The Dragon's Call", Uther told the people during his speech that twenty years ago, the Great Dragon was captured. Since he started the Great Purge after Arthur's birth, Arthur was twenty years old in this episode and turned twenty-one later in "Excalibur".

In "The Wicked Day", Arthur turned at least twenty-three, if not twenty-six, depending on how much time had passed within the seasons. If every season was approx. one year, plus the time gaps between seasons two and three and then seasons three and four, he turned twenty-six in "The Wicked Day". If a season didn't cover a full year buta few weeks or months, he turned in season four something between twenty-three and twenty-five.

It's safe to say that the seasons covber at least some months, because Arthur's birthday is either in spring, summer or autumn (green trees in both epsiodes of his birthday), whereas only a few episodes later it was winter already.

At the end of the show, Arthur was either twenty-six/twenty-seven or twenty-nine/thirty years old.