User blog comment:Emily Windsnap/How old are the Merlin characters?/@comment-174.52.49.106-20170621034600

They're all around 20 in the pilot, I would think.

We know that Arthur's mother died in childbirth, causing Uther to decree magic as evil and hunt down all sorcerers. Around this same time, he gathered all the dragonlords to capture Kilgharrah, and then went to have them executed, forcing Balinor (Merlin's father) to flee, in turn meeting Merlin's mother. In the pilot, we hear Uther say it's been exactly twenty years since the Great Dragon was captured - meaning all of that happened twenty years ago.

Like another user said, there was also a coming-of-age episode (meaning Arthur was turning 21).

So, I think Arthur's 20 in the pilot, and Merlin 19, maybe even 17/18 (which would explain why they call him 'boy' in the earlier seasons)

Hope that helped!