By 4th grade, students are able to associate different spellings with different word meanings. Since incorrect homophone usage can be a life-long spelling error for many, many adults, it makes sense to start as early as possible to TEACH the CORRECT USAGE of these special words! If you've every mistakenly used the wrong to/too/two or sent/cent/scent, you know exactly what I'm talking about. If we allow students to use homophones incorrectly until they are in upper grades, we've given them several years to reinforce bad habits NEEDLESSLY!