Average English speaker is fluent at about the age 12; the average Polish speaker is fluent in their language not until age 16. .
to whoever wrote this, I'd suggest a visit in any kindergarten, they might soon change their mind.
My son got fluent in Polish at about 3 and he's been fluently speaking it ever since, day in day out, approx. 16/24 and 7/7... and I don't mean to say that he's a genius ;)
I think the author of the article is confusing fluency with lexical competence.
oh, and 7 genders??? I seem to remember just 5.