Thinking over my experiences as a German tutor, my undergraduate German studies and the training I have received in teaching foreign languages, I've come to realize that there really is no such person who "can't learn a language" or "doesn't have the brain for it." Everyone has the brain for it, it's merely a question of motivation, and I will explain what I mean by this.
If you had grown up in a place where you had to speak two or more languages every day, you would have learned them, used them, and not even thought about "not having a brain for it". In such a situation you have no choice - learn or be excluded from the everyday life of your community. This is why, in language instruction, "immersion" is so highly valued, because you MUST learn, you WILL learn, and though it's a bit unnerving at first, you do learn so quickly that you don't have time to doubt yourself. This is the question of motivation - you are highly motivated in an immersion environment because you want to be accepted and included. Children learn languages quickly and easily for a number of reasons, but motivation is a huge factor - they are truly the most motivated of language learners, because they so much want to express their thoughts and be a part of the wide world opening around them.
Motivation is something we all must consider when we see our children or friends or fellow-students struggling with language learning; in my own tutoring I consider it highly important to MOTIVATE a student. Why should I learn this language? - so the student might be asking him/herself when plowing through grammar exercises or a difficult reading. It's a great question - are they doing it because it is required and this language seemed easiest? Reasons such as that make for unmotivated learners. What can we do for students who are unmotivated? We must move outside the "I have to do this and get a good grade", though that may be true. I try to make students see the beauty of a new language by teaching culture as well as language, drawing from my own motivation for studying German. I like German not only because I like the sound of it, but because I love German history. Does this student like Spanish literature, or Greek food, or Italian music? Does this student like to read about American colonial history? These are things that motivate - and just a few ideas to broaden a student's perception of language beyond the grammar book and into an exciting and wonderful culture that lies just on the other side of the doorway of language.