"They're only (age)" has never been a valid excuse

Saying a child is only (age) whenever they misbehave is both lazy parenting and neglect. Sure, their brains are still developing, but they will not properly develop without parents and other authority figures helping to guide their moral compass. Allowing children to break things or attack others simply because they're young sets a dangerous example which teaches them that they can get away with anything because they "don't know better". Later in life, when they're fully grown adults, they will continue with this idea and end up either beaten up or thrown in jail, with their only defense at their trial being along the lines of "I didn't know better". Teach your children right and wrong before the real world does.