Well, back in feudal times you were grown up with 16 if I remember correctly. Younger rulers were not unheard of either. It is how monarchy sets itself up for disaster. When the previous generation is dead, the heirs might only be 2. They still get placed on the throne, even if the power rests elsewhere. It very well may be that this coup was not of the brothers design, perhaps there was some kind of bigshot involved behind the scenes, waiting for a moment when brother and sister had a falling out and made use of the boy's anger to gain the realm. Not everything will be as it seems.