From when should I teach about menstruation and body changes?

The basic principle is to start telling them before the changes begin. For menstruation, prepare concretely from when body changes start to appear—often by around age 10. Emphasize "what to do in the moment" over "knowledge": practice the steps with real pads, prepare a step chart with pictures/photos, and use a calendar to give a sense of what's coming. Because sudden onset is very unsettling, convey beforehand that bleeding isn't an illness and "here's what to do when it hurts." You can also consult your regular doctor or an adolescent clinic.