I read in this revisionist history (the good kind) book Lies My Teacher Told Me that history professors regard their inbound charges as starting their higher education with a deficit. Kids have been taught so many lies, myths, and apocrypha posing as proven facts in their primary educations that to even begin writing on the white board of their minds you must first spend a few semesters erasing all the gibberish already written there.
English education is the same way here. Adults and kids would be different kinds of learners no matter what the content of their education was, but it just so happens that it’s a load of crap. Adults come into their first post-public education English class (fun fact: public education is called “duty education” here) saddled with at least 6 years’ worth of incorrectly taught grammar – which is sadly public school English’s best area – as well as deeply ingrained katakana pronunciation (I-moo goin-goo tsu-u go tsu-u za paa-ku) and a large vocabulary of creatively reinterpreted English words like “kanningu” (cheating on a test, which indeed could be described as but not called as such).
The preschoolers have far better pronunciation and grasp of social English than the adults – yes, because kids learn those things easier, but also because they never learned it wrong. I tell all my elementary schoolers what common errors to watch out for when they start junior high English and not to be afraid to correct their teachers, which they almost certainly will have to. The sad truth is that on any matter other than spelling or extended vocabulary, they already know more than their future teachers do.