The knowledge should not be forced but it should be out of willingness. I do not care how old my child might be but if he/she is interested in learning and understanding Bitcoin I will teach the child. Just the same way I won't force the knowledge on them it is the same way anyone who has the potential and wants to learn I won't deny being the mentor.
One way or the other I still wish at least one of my children would be willing to learn about Bitcoin.
When the child is off-age, as the parent who is interested in bitcoin, you can just call them together, even if it's just a single child you have. You can give them a child-to-parent kind of talk and share your basic knowledge about bitcoin with the child.
How they respond and the interest the child picks up from the teaching will determine if you should go further or not. I think we are now in an era where children are more exposed and know how to make choices on things they want even before they reach the age of 18.