The younger a child is when you introduce a language, the better. It's not that some can't learn (if that's the case, they wouldn't be speaking English) it's that we introduce a second language too late. Not only do we introduce a foreign language too late, but we don't have the opportunity to interact with native speakers every day.
Use it or lose it. It's one of the down-sides of having an efficient brain.
If the goal of public education is to turn out graduates who are productive members of society, then teaching non-English speakers English as quickly a possible (immersion) is the only approach that makes any sense. The product of bilingual education is frequently a graduate who is illiterate in two languages.
I can agree with the second statement of the title, but not the first. Saying that our kids "can't" learn a foreign language is basically saying that our kids are stupid or have learning disabilities. The ability to learn the basics of a language (foreign or not) is not part of our culture or heritage, but a part of our human fabric. The ability to communicate in multiple ways is one of the many traits that seperates us from lower animals. As a member of the military, I've seen people speak more languages than I can count, many of whom learned multiple languages informally. The ability to learn multiple languages is undoubtedly universal, but the WILL to learn subjects (any subject) in school can waver. I can understand the debate of whether or not we should be teaching foreign languages in school, although I am in favor of it. But telling our children that they are not mentally capable of performing basic human intellectual skills is not helping our kids, only hurting them