I don't think we can determine an ideal age, not all children are the same in the sense that they mature at the same age. I think children should be approached individually without forcing them to do anything, or rather, we should examine whether they have an interest in these kinds of things.
It would be ideal to ask children if they have heard of cryptocurrencies, if they know anything about them, and then occasionally give them information that can help them research things online for themselves. If they are interested and have additional questions, you will be the first to know, and that is the best sign that they are ready to learn more.
Cryptocurrencies (especially BTC) should be presented to children as an alternative to the existing financial system, not as a get-rich-quick scheme - unless you want to turn them into crypto gamblers.