User blog comment:OneofthePendragon94/The age of Merlin, Arthur, Morgana and Gwen./@comment-98.247.144.168-20190709071431

Arthur is 25 in season 1. In Season 1, Episode 6 (A Remedy to Cure All Ills), King Uther says to Gaius, "You've been here since Arthur's birth," and earlier he says "Gaius has served me well for 25 years," to Edwin.

If Gaius has served for 25 years and has served since Arthur's birth, then Arthur is 25.