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Does Starting English Earlier Really Help? What Second-Language Research Actually Says About the "Critical Period"

By Leon MoriguchiPublished May 15, 2026Updated May 18, 20265 min read日本語版あり
Audience
Parents of children 0–12 considering English-language instruction

TL;DR

  • ·The critical period for second-language grammar acquisition ends much later than commonly assumed — Hartshorne et al.'s study of 670,000 learners found that grammatical plasticity is maintained until around age 17.4, substantially revising the popular "before age 10" framing
  • ·When total exposure time is controlled in research, the advantage of an earlier start disappears — earlier starters achieve more because they have accumulated more hours, not because they started earlier
  • ·Japanese school English instruction amounts to about two class periods per week, far below what acquisition research identifies as necessary; designing meaningful contact hours at home — enjoyable, repeated, self-motivated — matters more than the starting grade

Contents

  1. Lead
  2. Background: What Is the Critical Period Hypothesis, Exactly?
  3. Part 1: Where Does the Critical Period End?
  4. Part 2: Exposure Amount and Quality Matter More Than Starting Age
  5. Part 3: Designing Exposure at Home
  6. Putting It Into Practice
  7. Summary
  8. References

Lead

Among parents of young children, the anxiety is familiar: we need to start English early, before the window closes. The idea that children's brains are more plastic, more absorbent in the early years, feels intuitively correct — and it has the appearance of scientific backing. Early-enrollment English conversation classes, English picture books, English-dubbed cartoons: parents encounter these choices constantly, and the framing around them usually implies urgency.

But second-language acquisition (SLA) researchers have been working on this question for more than 40 years, and the picture that has emerged looks somewhat different from the popular narrative. The emerging consensus is that when you start matters less than how much exposure you accumulate, and in what form. That conclusion deserves to be taken seriously.

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