I learned German informally (by speaking it continuously at home with said wife) as an adult and am typically considered fluent BUT: A> accent is atrocious and B> any fluency is rich and deep within a few domains; outside those domains I flounder like an experienced beginner (which is what I consider myself).
There are varying degrees of "fluency" but I certainly know plenty of people whom I would consider fluent in a language they learned as an adult. Articles that claim that it's impossible have always puzzled me.
It probably wouldn't be unreasonable to think of many native speakers of English in similar terms with regards to English fluency.
There is so much knowledge out there now that you basically can no longer be a Renaissance Man. We all know a lot about some things and next to nothing about others.
My German is more like a Hawaiian island :-) -- very expressive for discussions around the house, politics, earth moving equipment and farm animals (the latter two due to childrearing). I am fine going out to the shops for groceries and calling to complain when the TV cable went out. But at parties I can only talk about so much; discussions of religion, negotiating contracts and the usual business day are utterly beyond me.