age of actors

It is frequent that actors are older than their character. Here, Joffrey is 19 or 20 and doesn't look younger. The other actors, the scenario, the dialogues etc. all correspond to youngsters about 18 y.o or more. What is the point in presenting Alex as a 15 y.o. boy ? (The competition is for cadets, 16 max).
This said, it is a very good movie and the three main actors are very good indeed.