Some people argue that it's harmful for children to use the standard algorithms of arithmetic before they understand how and why they work. Supposedly "premature" use blocks future understanding.
At 3 years, most children studied produce the focal cognitive terms think and know in the course of everyday conversations, at least occasionally...It is not until about 4 years that children appear to use these terms to indicate specific mental states, distinguishing between the meanings of think and know on the basis of certainty...It is not until the early school years that tests of comprehension show clear discrimination among the terms think, know , and guess. And even in the late elementary years, children do not demonstrate understanding of the full range of distinctive meanings of know.
The acquisition of meaning of abstract terms such as mental-state words is best conceptualized in terms of acquiring meaning from use.