Second Language Acquisition Studies

How do people learn a second language? Does it matter how old they are when they begin to learn a second language? Does it matter whether or not they already speak a first language? Does it matter under what circumstances they learn a second language--in the classroom as opposed to the "real world," or in Iran as opposed to an English speaking country? Does it matter how the new language is presented to them? Does it matter what kind of a person they are, for example, impulsive or reflective, motivated by the need to advance professionally or by the desire to learn more about speakers of the target language? Can one ever learn a second language and pass for a native speaker of that language?