As was said, if varies from child to child. However, I find there are two things that children crave, even though they might squawk for show. Rules and challenges. They need to know what the rules are upfront. Doing homework in elementary school is something that should be enforced. If there is homework, they need to do it for their class. Additionally, they will have homework throughout their school career so they might as well learn that is something that is not optional.
As for when to introduce tougher topics into their lives, as with all things, you start with the very basics. Slavery was mentioned. When you first talk about the Civil War, you need to bring slavery into the picture. It is pertinent and needs to be discussed. You don’t have to go into political rhetoric or things like that, just the basics. It is those basics that later classes can build on.
As for giving a child responsibility, I think the sooner the better. They learn how to do a task and get a sense of accomplishment when it is done correctly. I was mowing the lawn at about age 10, babysitting at age 11, helping mom cook from about age 8, complete with washing dishes. Giving responsibility is not passing off work to the kids for free labor. It is to help them learn how to do things and why it is important. This requires, from a parent, the patience to teach the child, the ability to correct poor performance without degrading the child, and the ability to let the child fail on their own, offering help only when it is asked.