Excellent idea and a great question. Since bitcoin is part of our world economy now, it would make sense to teach kids at the age when they understand what money and credit card are. My guess is that it would be around 10-12 years old, depending individually on child development. Cryptocurrency in general is a very new technology. Kids will be the ones living "in it", unlike those who remember the times "before" and "after". The more honest you are with your kids, the better they understand what you are explaining to them. Draw pictures, explain in simple words, be open about this issue and I am sure your kids will understand the point. One of the best examples on blockchain technology explanations I have seen is this.
Let's say Bob, Julie and Andrew want to trade services (or goods) and want to write down who owes how much bitcoin to each other. Make a piece of paper for each one of them.
Let's say Bob has 10, Julie has 5, Andrew has 20. Andrew mowed the lawn and now Bob sends him 5 bitcoins for the service. The change happens on Bob's piece of paper, Julie's piece of paper and Andrew's piece of paper. This is how blockchain works - everyone owns a piece of paper with accounting information for everyone. 51 % of those "pieces of paper" decide if a transaction happened or not. I really liked this analogy to explain blockchain to someone not familiar with bitcoin.