In Ender's Game, are Ender's actions at the beginning of the book appropriate for his age?
Definitely not. Of course, this is the reason why Ender is so special and why he has been selected as the possible future saviour of mankind. Note how he acts in the fight with Stilson. He shows an understanding of human psychology and a willingness to act on his keen intelligence that is above and beyond nearly all six-year-olds. Let us examine his thinking:
Ender knew the unspoken rules of manly warfare, even though he was only six. It was forbidden to strike at an opponent who lay helpless on the ground; only an animal would do that.
And that is of course precisely what Ender does. He cooly and calculatedly kicks Stilson's supine body hard, twice, in his groin and ribs. Of course, he knows that he has to hurt Stilson so badly that nobody else would want to bother him to ensure his survival. This is definitely not the behaviour of your average six-year-old.
Of course not. Unless, children normally analyze their bully's psychology and go out of their way to beat them not only physically but mentally. Ender says that he knows the only way to get the bullies to stop is by beating them so badly that they will never dare to hurt him again. Even after beating Stilson, he kicks him again on the ground to ensure that other's see Stilson as an example of what could happen to them.