I think kid who is old enough to understand the concept of currency. Kids playing games on cellphones or playing cardboard games like monopoly do understand buy, selling and barter concepts. When playing games there are lot of upgrades for which payments are required. and games like monopoly which are literally build on effective economics helps kids to understand the currency in better way.
Once kids are familiar with the concept then starting with the digital payments, plastic money and then crypto currencies will make it interesting and easy to follow.