I'd say it's the capacity for what they can deal with. They've been conditioned to violence since they were toddlers. Cartoons are violent. Other kids will shove and push. They will have fights. By the time they're 11 they have a good understanding of violence, the pros and cons, what's too much and what's ok.
Most do not have the understanding or capacity to understand sex because they have never experienced anything like that yet and still won't for a number of years. We as parents shield them from this things because out if context, it will completely confuse them and they might draw the wrong conclusions from it. Obviously as parents we help guide them through those topics but a 10 year old may hear you and not understand what you are telling them about the subject. They have a better capacity to understand when they get to 12 or older.
That's my take on it anyway.