One will never reach the level of a native speaker, but attaining a level at which native speakers are at ease during casual conversations is possible.
Got to disagree with this, having known several folks in person and online who've done it. Online, examples include the guy from
www.alljapaneseallthetime.com and the dudes from
www.antimoon.com off the top of my head. Ditto this example from Stephen Krashen's website:
http://www.sdkrashen.com/content/articles/what_does_it_take.pdfBasically, the way you do it is to surround yourself with native materials 24/7, and from there on, it's just a question of logging the hours. A buddy of mine did this and got to the point where he passed the proficiency test for being able to teach in the language (Spanish) as a public school teacher, could speak the language fluently to the point where native speakers had trouble believing he didn't have native parent speakers, and decided to raise his newborn daughter bilingually. And this was all in less than two years of daily work. The guy from AJATT did it with Japanese in about the same amount of time.
It takes a lot of work, though, as well as a rock solid level of dedication. You essentially have to trade everything you do in your native language for everything in the second language, which is not a sacrifice most folks are willing to make. My buddy stopped listening to music except for in Spanish, only listened to the radio in Spanish when driving, and used headphones whenever using his computer at home because he was always listening to online radio in Spanish. He only read books in Spanish (children's books, teenage books, adult novels), and watched several telenovelas (Spanish soap operas hundreds of episodes long) start to finish. In other words, he surrounded himself with the language the way the guys above describe (that's the method he followed). In the end, though, it got him where he wanted to go. And in true Mustachian style, he barely spent any money in the whole process.
I can vouch 100% that it's possible to get to the native level in a second language as an adult, and it's not something that takes the rest of one's life to do. However, just like retiring early, it's something that most people believe is impossible because it involves changes they aren't willing to make.