My teen's body is growing but their understanding is younger. How should I teach them?

Because the body grows on a chronological timeline, match the content to adolescence while matching the delivery to the child's level of understanding. Rather than abstract explanations, use pictures, photos, real objects, and step charts to convey "when, where, and how to do it" concretely, briefly, and repeatedly. Give a heads-up before things like menstruation, ejaculation, or body-hair and odor care happen, and build them in as a routine to reduce confusion. Teach the public/private distinction with concrete places too. Rather than aiming for perfection at once, build up in small pieces, situation by situation.