I remember when I was in school and they basically told us to forget all that we had learned since we were now going to learn the metric system. I was happy as a pig in shit because the metric system actually made sense. Even Coke came out with a one and two liter bottles.
Then I graduated and went to work in the real world which meant my parents contracting business which was not using the metric system. So I had to teach myself fractions, feet, and all the other fun things that goes with the American system.
I can do both, guess I'm bi-measurement.
In answer to your question I'm not sure of the exact date, but I seem to remember it was sometime around 1970.