As prettymuch everyone has already said, as children many things are censored for the guardians. They must learn all of the right values and morals while they are young. This is in order to shape them into what a guardian must be. Once they have been properly "shaped" so to speak, being influenced to other things that are not as "good" as what they have been brought up with won't affect them. Now they must begin to prepare for their real work, and it only makes sense that they should be exposed to other things in the world to get a better understanding of the people they would be ruling over.