"The principle of least action could be taught to early on in education." What is the baseline for "early on"? The students at the same grad level are not all the same. There all have their own educational back-ground. We can teach physics ideas for kids if it was intriduced in an appropriate way and with enough background information. But "appropriate way" will depend on student's background knowledge. I believe there is no such thing as "One rule for everyone". Yes. It is possible to teach the principle of least action even for children. But, we should not force all the children to learn it.