I think the problem people are facing in answering your question is that much of what is learned in school is content, not skills ... and as such it doesn't translate into levels very well. Reading is primarily skill-based, so it's easy to measure a KG'er and say "he's at a 4th grade level." That's why reading levels are often measured in children.
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With other subject areas he'd only be at a 4th grade level if he'd been taught 4th grade material. Social studies, science and second-language subject areas are primarily content-based (math has a bit of skill-based learning in it too). If your child hasn't happened to be exposed to information about xylem and phloem, he's got a gap in his 3rd grade science knowledge in my jurisdiction. Yet he may know all about evolutionary changes in the hip structure of pre-dinosaur lizards, something that might not be taught until college in most places. Does that make him "not yet at a 3rd grade level," or "at a college level"? Neither, really. I don't think you can talk about being at a particular level in content-based subject areas. All you can say is that he is "capable of handling content and academic expectations at a ____ level." So for example I have a 2nd grader who is working capably through a 6th grade science curriculum: she reads the material, answers questions independently and accurately and scores near perfect on the tests. Because of that I would probably say she is "at a 6th grade level" but only in the sense that she is clearly capable of handling the course material at that level. If she hadn't worked through the curriculum I couldn't have said anything meaningful about her science level.
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Miranda