When time will come they will learn buy themselves, you just say them that such a thing exists 
We cant say when or what age they must need to know bitcoin but for me there is no age limit as long as they really wanted to learn about this then we can say that was really the right time. They must need to learn on how or what really bitcoin all about for them to know how future lives works. Bitcoin is really important to teach them because it really can help them on learning how to save money. So must treasure it.