One could speak volumes on that, whether from the experience of having to grow up too fast to seeing that ignorance is the other side of innocence. Children can be cruel, especially in group dynamics while learning the ropes of social roles in acceptance versus individuality.
So are youth and innocence two separate things? Yes, however, the law would argue that until a certain age a juvenile is not mentally capable of comprehensive moral judgement. Having studied child psychology and having been a child myself in difficult circumstances I would have to partially agree and disagree. Some children come to understand some things more quickly than others, but those laws are placed as a guideline intending to protect the fact that children are still developing human beings. Youth and innocence are neither mutually inclusive nor exclusive. Each situation and individual would have to be assessed.