I never said that you couldn't learn a language as an adult - in fact I said the opposite. However, regarding the research, I understand the sentiment Malenky, but how can modern scientific research findings/theories be "rude" (the 80s and 90s weren't the dark ages)? They are just findings based on research.. Obviously that changes all the time, but, again.. rude? No.
I, and everyone I know (even the Dutch, funnily enough!), think of it in the same way as, for example, a white person I can't ever become truly black, however much they try (clunky example, but you get the point). Like I said, fair enough, if there is now new research to show that the cut-off point is not universal, but it doesn't mean that there is clear consensus on the subject of there not being one..
I get told that I speak like a native quite often, and people are amazed to find I'm not from Britain, but as soon as I get home and relax I can let it "all hang out" a bit more, so I do have a tiny bit of an accent. Certainly if I'm furious 
I would say that one part of speaking a language is in-depth knowledge of the culture, which a foreigner might lack. I certainly don't get all the references DH trots out. We had completely different telly/music/popular culture in general in my country. Historical references are hard to catch-up on. I'm trying, but still don't "get" things like what's good/bad about the Carry On films. I "get it, but at the same time I don't IYSWIM. I'm now falling off the wagon when it comes to the latest Finnish cultural references (have been here for nearly 10 years), so might fall into the gap in-between languages.