Crown Prince Thomas Crowne was perfectly fine waiting to join the knighthood until he was older. His attempts at escape may suggest otherwise, but he truly didn't want the classification of adult that the knights gain. 
Then his mage trainer High Mage Lord Odin Periwinkle was removed from the castle for an issue that wasn't true. 
Without his trainer's support, Thomas decides it's better to gain distance from his parents' foolish actions and thoughts by joining the knighthood. Fifteen is the minimum age and even if his father tries to refuse him, he'll figure out the way. Because living as the child of a king that punishes his top mage for no reason is not his desire at all.

Chapter 121 - Break