There's no specific age to teach them about crypto, well, you just let them know that it exist, then you can tell them how to earn it in the future. I think it's good if he/she is in high school before starting to engage in the community.
The age is depending the child. Its not the same betwen one children with other when they are ready.
However, 18+ years old is enough to start learning bitcoin. Tell them the risk first before the profit.