Proof: The foundation of mathematics
Primary students are capable of arguing deductively, as shown in the video Addition of Odd and Even Numbers.
You can download the Addition of Odd and Even Numbers video transcript.
For secondary students, geometry is a common area for proof exercises. However, many Number and Algebra activities also lend themselves to formal proof. For example, the activity Magic Vs is a proof activity from the Nrich website that is suitable for years 4–10.