It depends on the child, but I figure if they're old enough to play video games, they're old enough to learn a bit about cryptocurrencies. You don't want to just turn them loose, though, as they will need parental guidance navigating the various crypto-sites on the Internet. There are a lot of ads for scam sites and you sure don't want them getting lured into those. But you can get them a wallet, and maybe pay them an "allowance" in a crypto of your choice (I'd suggest DMD Diamond). You can show them the basics of trading, and you can explain the technology at age appropriate levels. You can also let them know a bit about what kinds of crypto-activities you do.