I completely agree zmjezhd.

Any adult who studies for long enough will learn the language. And if what you say is true that "these people [who succeed] are few and far between", it is due not to a physical impediment in the brain, but insufficient determination. In other words, few succeed because few are prepared to make the effort required to succeed; not because few can succeed.

The marked difference in difficulty, together with my observation that rote learning a second language as an adult is worse than useless but that mnemonics are very effective, was what prompted me to wonder about possible differences in brain dynamics.

Wikipedia has some interesting information on that question.

Critical period hypothesis
Language acquisition: Critical period hypothesis