It’s not just about “burning off aggression” it’s about learning to handle someone elses aggression when you can’t get away and will not listen to anyone trying to talk calmly.
Like in Crown Heights riots for example, if thugs shouting “Kill the Jews” attack a Jew, he needs to know how to defend himself, whether they have sharp objects or weapons or whatever.
The earlier kids start learning this the better they will be able to handle themselves when the time comes.
I remember in Crown Heights many years ago, someone in an apartment building with a mix of Jews and Gentiles, wrote to the Rebbe saying some non Jews were physiaclly harrassing his boy.
The Rebbes response for what I heard was “teach your boy to fight”.