Once somebody is able to drive a car, he should be able to understand bitcoin as well and handle it in a responsible way.
The age to get a drivers licens differs from country to country, but as far as I know it's between 15 and 18.

In general I would not teach bitcoin in schools.
Imagine a teenager was tought about crypto currencies at school, opens an account and loses all his money.
I am sure that in case of a lawsuit the school would be held responsible and it would have to make up for the loss.