User blog comment:OneofthePendragon94/The age of Merlin, Arthur, Morgana and Gwen./@comment-14.200.40.121-20121116062141

I think Merlin is 19 at the beginning of S1 Because I read it somewhere, Arthur is 20-21 like you said, Morgana 18-19 like you said but I have a feeling Gwen is at least Morgana's age coz she had been her maid since they were small, so Uther must have wanted a responsible, slightly older girl to be Morgana's maid.