Although, for some people, the only thing that tells them right from wrong is someone else who talks about morality; there are people who have a natural kind of empathy and compassion that is, in itself, their moral compass (regardless of what any "written rules" are about morality). Some morality may come from a person's reasoning ability and common sensed. For example, the sixteen-year-old who decides she won't sleep with every date may be aware of the risk of serious, future-damaging, consequences of such carelessness. Enough self-esteem, self-respect, and respect for others can also play a role in a person's refusal to do something s/he knows wouldn't be the wisest or healthiest for her or someone else.
All that aside, I pretty much believe that empathy and compassion and a wish not to hurt anyone or anything, as well as a nurturing and protective instinct to others, are developed in the early childhood years when the brain is being developed. I believe that if those things are well developed from early childhood on, that's when there can be more of a need, for some people, to be told right from wrong by someone who is respected. Even when empathy, compassion, and a nurturing instinct are well developed in early childhood, though; I believe there's still the need to support and reinforce that by talking to children about what is right and wrong.