I would say that any age is good, but if children understand fully what money is and have experience of earning money themselves. Currently, cryptocurrency is more about investment, than transactions, technology, anonymity. Parents should be ready that "making money" will be the only thing that will interest children in crypto, when they start to learn that. Everything about starting own project, blockchain, testing - they will find all that boring and wont focus on learning. They will only focus on "if they buy X, when they will become rich". If a parent is ready to discuss all that, ready to be responsible for unfulfilled dreams, fails in trading, wrong expectation, then a parent should start introducing crypto to children. Be ready to cover their losses from own pocket.