I've never even seen any claims that there is a significant difference in language learning ability between teenagers and adults. There is strong evidence that

I've never even seen any claims that there is a significant difference in language learning ability between teenagers and adults. There is strong evidence that very young children (younger than three years) can pick up the phonetic aspects of languages in a way that adults simply can't and people often simplify that to "young children are better at languages". Given that this guy's key message seems to be about the importance of actually using the language (over taking classes or reading textbooks) I'm not sure that you actually disagree with him.