The fact that he commanded the respect and loyalty of all those officers, who were older and more experienced, tells you a lot. Not a single one of those men, who served under his father and knew Philip's caliber, would have allowed an immature, incapable teenager lead them. He would have been murdered quickly, as it happened many times to the lesser children of great emperors/kings/etc.

At those levels, loyalty and respect are very much something you _must earn_. It is not given.