Part of the confusion, I think, is that people unnecessarily require "perfect pronunciation, hearing, and intonation" in their definition of "fluency". You mentioned that your wife is fluent in French, but does not have native-like French "hearing". Perhaps she also speaks with a non-native accent. That doesn't detract from her fluency. Arnold Schwarzenegger, for example, would be considered to be fluent in English even though he speaks with a heavy accent.
Babies and pre-pubescent children do indeed have a distinct advantage when it comes to learning sounds, tones, and pronunciation for a language [1][2]. It's also been observed that children are more likely to sound native (in language 2) if they move to another country and learn that second language prior to puberty (see again, Nagle [2]). So, gbog, if your child speaks/hears French, Chinese, and English regularly before hitting puberty, he/she will likely sound native in all three languages. But this advantage doesn't nullify the claim that adult learners can still become fluent in a foreign language. It just means that adult learners might never sound exactly like a native speaker, even though motivated learners can achieve near-native pronunciation in their target language [3].
[1] Babies are born with the ability to hear/distinguish between the sounds of all world languages, but this ability starts to diminish around the age of 10 months (pp 39-40, The Bilingual Edge, King and Mackey, http://www.amazon.com/The-Bilingual-Edge-Second-Language/dp/...).
[2] "Critical period research clearly demonstrates that the probability of a near-native L2 phonology rapidly diminishes as we age", Charles Nagle, "A Reexamination of Ultimate Attainment in L2 Phonology: Length of Immersion, Motivation, and Phonological Short-Term Memory", http://www.lingref.com/cpp/slrf/2011/paper2913.pdf
[3] "The results of this study highlight the fact that learners appear to be able to achieve near-native pronunciation without significant formal instruction in pronunciation which perhaps also evidences the role of implicit learning in L2 phonology...", Nagle, http://www.lingref.com/cpp/slrf/2011/paper2913.pdf
Babies and pre-pubescent children do indeed have a distinct advantage when it comes to learning sounds, tones, and pronunciation for a language [1][2]. It's also been observed that children are more likely to sound native (in language 2) if they move to another country and learn that second language prior to puberty (see again, Nagle [2]). So, gbog, if your child speaks/hears French, Chinese, and English regularly before hitting puberty, he/she will likely sound native in all three languages. But this advantage doesn't nullify the claim that adult learners can still become fluent in a foreign language. It just means that adult learners might never sound exactly like a native speaker, even though motivated learners can achieve near-native pronunciation in their target language [3].
[1] Babies are born with the ability to hear/distinguish between the sounds of all world languages, but this ability starts to diminish around the age of 10 months (pp 39-40, The Bilingual Edge, King and Mackey, http://www.amazon.com/The-Bilingual-Edge-Second-Language/dp/...).
[2] "Critical period research clearly demonstrates that the probability of a near-native L2 phonology rapidly diminishes as we age", Charles Nagle, "A Reexamination of Ultimate Attainment in L2 Phonology: Length of Immersion, Motivation, and Phonological Short-Term Memory", http://www.lingref.com/cpp/slrf/2011/paper2913.pdf
[3] "The results of this study highlight the fact that learners appear to be able to achieve near-native pronunciation without significant formal instruction in pronunciation which perhaps also evidences the role of implicit learning in L2 phonology...", Nagle, http://www.lingref.com/cpp/slrf/2011/paper2913.pdf