Proficiency
I know that this is a very broad question, and the answer has much to do with effort, but: Is it possible to become as fluent in a language learned later in life as it is your 'native' language? Can you reach a point where you can express complex thoughts equally well in both languages?
I would guess that it is a massive undertaking to develop as large a vocabulary in a second language as you have in your first.
7 Answers
I came to the conclusion a long time ago that I could never become truly "bi-lingual" unless I managed to forget about half of the English that I know. I am reasonable comfortable carrying on a conversation (or reading books) in Spanish (and when I was in better practice, in French, as well). By some definitions (not mine) that would make me 'fluent' (though of less than "native proficiency").
What did it for me was looking around the kitchen at all the cooking implements (when I was, perhaps, in my twenties) and realizing that in English I knew such words as 'colander', pressure cooker', 'garlic press', 'double boiler', 'spatula', 'egg-beater' and a host of other terms. With a bit of reflection, I added the names of dozens of tools (hardware items) that I had learned as a child. I include in the list "sash weight" (a large chunk of iron that facilitated opening and closing old-fashioned windows) It occurred to me that I had learned all of these words as a child but had no idea how to say them in Spanish or French. It further occurred to me that I had never heard/read them in Spanish or French and that, in all probability, I never would. Many of these are words that I might have occasion to use once in 5-10 years in English but they're there, tucked away in the back of my brain, relics of my childhood.
Professor Higgens of "My Fair Lady' fame (or, if you prefer, the corresponding character in Shaw's "Pygmalion") astounds his listeners by correctly predicting (on the basis of a few utterances) that a certain person was born in X part of London, but after the age of 10 (or some such) was raised in some other part (Y) of London and, as an adult lived in region Z of London. There is an underlying factual basis for this sort of thing. Studies have shown that certain words (or classes of words) are typically learned at various definite stages of life. That is to say, there are a bunch of words that you probably learned between, say, the ages of 5-10, others that you learned as a teenager and, still others, that you learned as an adult. The fact that you learned a particular pronunciation (or expression) can be predictive of where you learned it (which, in turn, suggests where you were living at that time in your life).
Even the term "native speaker" poses problems (at least, when it is interpreted as meaning some sort of "expert" in his native language). Mostly, the term should suggest a competence in recognizing "grammatical"/"ungrammatical" utterances. When it come to issues of vocabulary, the situation changes completely. The skilled carpenter/plumber/car mechanic does not know/recognize the specialized vocabulary of the brain surgeon and neither understands the specialized vocabulary of the astrophysicist (nor he, their vocabulary). The astrophysicist may not be able to explain the difference between an 'adjective' and an 'adverb'. They each have expertise (in their respective areas) but this expertise does not qualify them as experts in the language.
I think it depends. If your first language is English and you move to Spain in your twenties, you might be able to become fluent in Spanish, and you may be able to speak like a native. But if you live in the US and you try to learn Spanish you will never be as good because you are not surrounded by the language.
I believe it is, especially if you have the opportunity to immerse yourself in the language.
For example, my native language is Bulgarian, but I have been living in the States for 10 years, and speak both languages equally well. In fact, my Bulgarian is not as good as my English anymore since I communicate almost exclusively in English.
If immersion is lacking, it is vastly more difficult but not impossible. The key is to communicate in the language.
My college Spanish professor is "natively" fluent in Spanish, French, German, Latin, and Mandarin (Chinese), along with speaking three additional languages, even though he only started out knowing English. Did I mention that he's been legally blind since he was 13? If he can learn to speak 5 languages perfectly, then you can learn Spanish. Just keep motivated and keep practicing! 
This has been said before but I'll say it as well. The key is immersion. Unless you're a genius, your fluency will be proportionate with how much you use it. Some of that depends on your environment and some of it depends on you. If you are a native-English speaker and you move to Spain or work at a hospital in a Latino neighborhood, or are married to a Latino/a you have the opportunity to become fluent in Spanish. Whether you do or do not is up to you. (Unless nobody speaks English and then you don't have much of a choice.
I think that it is easier than ever to become fluent, of course depending on how much time you put into it. There is so much information available on the internet for you to access and practice with and you could even speak to native speakers on Skype or set up lessons by way of Skype. Of course if you live in the US you can probably quite easily find one-on-one lessons locally and then use your vacation as well to go somewhere and take an immersion course. I have a group of 'seniors' learning Spanish and some of them have made amazing strides.
Well, personally I think practically anything is possible if you work hard enough and you really want to do it. Así, sí, pienso que esto es algo que todos pueden hacer, si lo quieren mucho. ¡Buena suerte y sigue así!
