Violence is much easier to deal with as a concept, and the morality is pretty straightforward. Very young children can be taught to resolve disputes without resorting to violence (golden rule, anti-bullying, etc). Slightly older children can easily recognize the difference between actual violence and fake violence in cartoons, or movies, etc. Dealing with sexuality requires more maturity, and is a lot more personal than violence (which the majority of people in liberal western democracies aren't actually exposed to frequently).
