Research on the mastery of American Sign Language in deaf students seems to support the rule,

Question

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Research has shown that deaf children who are exposed to and begin learning American Sign Language at an early age have better language and literacy outcomes than those who start learning later. In particular, early exposure to American Sign Language has been linked to better speech and reading abilities in later years. Therefore, the rule "the earlier, the better" is supported by research in this area.

Research on the Mastery of American Sign Language in Deaf