2% of adult learners of second languages learn to speak without an accent. This happens even if the person was not exposed to the target language as a child.

2% of adult learners of second languages learn to speak without an accent. This happens even if the person was not exposed to the target language as a child. If accent can only be learned in the critical period, then we need to have an exception for those 2%. My experience has been that people get to a "good enough" level in a second language and their motivation for working hard goes down dramatically. Children, on the other hand, have a lot of incentive to work hard on their language every day. If you've ever known a child with a speech impediment, you can see how tortuous it is to speak differently than your peers. The other thing that colours my judgement is that every person I've ever met that has tried consistently to remove their accent has succeeded. However it's a 365-day per year, multi-year project. I've met very few people willing to undergo that experience.