Book VII of Le Morte D’Arthur describes Arthur’s coronation. What’s interesting here is that he “took the sword between both his hands, and offered it upon the altar where the Archbishop was, and so was he made knight.” Most kings don’t get knighted. They’re crowned, and that’s an end to it. With Arthur, the idea of knighthood is inextricable from his character. This is not merely a king, this is a knight-king, who holds to the chivalric code, someone of divine character.
This also clues us in to his age. Kay, his foster-brother is a knight. We have no definite age for Arthur, but passages in the book refer to him as “young Arthur,” so we can conclude that he is not yet of an age to be knighted. The path to knighthood was strictly controlled, too. At age seven, a boy can become a page. At 15, they become a squire. Finally, at 21, they’re eligible for knighthood. Since it is a young Arthur, we’re likely looking at someone in the age of a squire, someone who is 15-20 years old.
The pulling of the Sword from the Stone also tells us the likely age as Arthur is fulfilling the duties of a squire for Kay by running back to get his sword. We could also speculate that he’s at older edge of this range, coming up on 21, simply because the archbishop so readily knights him. This might even be a clue that he was already of age, but did not have any of his own lands. This is about as exact as we can get, especially considering that Malory’s book was written hundreds of years after the initial stories of Arthur. Of course if we want to throw speculation out the window, we look to Geoffrey of Monmouth, who tells us flat out that Arthur is 15 when he was crowned—however, there is no mention of the Sword in the Stone from Geoffrey of Monmouth.
So we have a knighted, boyish king. While the knighthood does point to someone who will follow the chivalric code and do his best to “stand with true justice from thenceforth the days of this life” we also have to remember that knights are in military service. The Stone Sword foreshadowed war, and the knighthood guarantees it. The best that can be said is that Arthur will do so in the name of justice and morality (given the divine nod), but bloodshed is coming. Young Arthur is a warrior king.