I've always been curious to know how old are the members of "The Quartet" (id est Merlin, Arthur, Morgana and Gwen). I read many posts and comments on the wiki, but I want to show you my calculation.
From the oldest to the youngest:
Arthur should be the oldest of the four. He is so protective... also, Eleonora De Angelis (who translated the scripts from english to italian langage) stated in an interview that english scripts had Arthur as the oldest of "The Quartet". His actual age is easy to calculate, because the Great Purge ended 20 years before Series 1, meaning that he is around 21 in Series 1. There has been eight years between Series 1 and Series 5, so at the start of Series 5 Arthur would have been 29. Assuming that 1 year has passed between Arthur's bane and the diamond of the day, Arthur is 30 when he dies!
Basing on Eleonora De Angelis statements, Gwen is younger than Arthur, but I think she is slightly younger, I think she is 20 in Series 1. So she is 29 when she is crowned queen of Camelot.
Morgana is younger than Arthur, as confirmed by Eleonora De Angelis. She should be at least 2 years younger than him, because Uther and Igraine had an affair after the end of the Great Purge. She should be 18-19 in Series 1 and 28 when she dies.
Merlin the youngest. He is 17-18 in Series 1, because the introduction had him as a young man only in Series 4 and Series 5. He is 27 at the end of the show.