At what age you can vote, see NC-17 movies and the like are all arbitrary lines. I know 30-somethings who can’t manage bank debt and I know 13 year-olds who can easily handle discussions on abortion.
I agree with Ahunter3 that the tools for debate and critical thinking should be taught in high school and beyond. Heck, much of this can start being taught in elementary school. “Why do you feel it is ok to shove Timmy on the playground.” You might not get an eloquent answer from a 5[sup]th[/sup] grader but you can start the wheels spinning when the child begins to realize he simply can’t come up with a convincing reason on why shoving people is a reasonable thing to do.
I think it is incumbent on any school to inculcate the fundamental virtues of honesty, fairness, and justice. All three of these are essential to the operation of any society (a society where dishonesty, unfairnessness, or grave injustice are the norms will not last long). We cannot expect our children to have a grasp of these fundamental concepts if we wait for them to become adults to expose them to them.
A fourth virtue I would like to see taught is courtesy. I don’t see courtesy as being as essential as honesty or fairness, but a discourteous society is simply that much less pleasant to live in than a courteous one. However, this is not intended to mean an education in the Standards Forms and Practices of Etiquette that many people think of when you say “courteous”, but instead the broad principles underlying those, such as respect for the personal privacy and individual autonomy of other people, the obligation to repay kindness with kindness, and the general principle that other individuals are entitled to be treated with respect.
And I believe that high school is far too late to expose children to these concepts. This education should start as soon as reasonably possible.
As to “hard topics”, I don’t expect to teach small children about homosexuality or abortion. But I think we should arm them early with the tools they need to come to their own opinions on these topics as adults, without having to be told what they think by others.