[Complexicate]

According to Montessori theory, it's ages 6-12 years old (the elementary years). That's when their mental development is naturally focused on fairness, what's right/wrong, social groups, justice, rules, etc. They begin to show a genuine interest in others, locally and globally. Role models play a huge part in developing their values at this time.

Piaget agrees, but puts it at 7-11 years old.

Both say the beginning and ending ages are an approximation and development occurs throughout, but that's the developmental stage when the brain is particularly sensitive to it.