My son is 4 and he's learning to read and write with a couple of $2 workbooks from Target, flashcards we made from 3x5 cards, some blank paper and crayons. Total cost about $10.
Short of a clinical condition, there is NO damn reason to be illiterate. Even in flat-broke districts, there are after-school programs, clinics run by non-profits and churches, free workbooks from libraries, etc...
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