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.
At what age it is not important to teach your children to encrypt. Everyone is not interested in the same points. If children are not interested in encryption, then release their nature and refrain from depriving them of innocence and happiness.