From about what age, and how, should I teach about private zones?

Private zones (the parts covered by a swimsuit, and the mouth) can be conveyed little by little in daily life, starting in early childhood when you begin teaching body-part names. Repeat, with short words and pictures: "this is your own special place," "don't show it or let others touch it," and "if something's wrong, say 'no' and tell an adult." Child-friendly materials like the NSPCC "PANTS rule" help. Explain exceptions concretely, such as a doctor's exam or bath time. Adding content to match age and understanding keeps it manageable.