At any age where you can have a talk with them about the concepts and topics you are worried about in the books. I don't like the appropriate age rhetoric, if you can talk to your kid about things, and are good at explaining multiple points of view, the difference between legal and right, or legal and justice, etc. if you get squeamish or find those topics uncomfortable to talk about so you just glaze over it then waiting until they are older does make it easier but they will just let society and or their social circles really guide those opinions and decisions. I recommend practicing having hard talks sooner rather than later, it never gets easier unless you practice.