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.
Children today are more aware of what is happening on the society, they are expose on things which is seen and learned in the internet, even a grade school student knows how to use gadgets cellphones and etc. You might be surprised that your children already know basic knowledge on bitcoin, without telling it to them. Remember that bitcoin and digital currency is for their generation.