I didn’t have time to look closely at this yet, but it might interest people here.
I have read that young (age 6 and under) children learn foreign languages faster than adults because they naturally use the strategies they used as infants learning their native language. For instance, making the common grammatical error of assuming all verbs are regular.
I disagree with the “critical period” hypothesis, because as far as I know there isn’t a cutoff point for learning a new foreign language to fluency - young children just become fluent faster than adults. And mnemonics can help adults become fluent much faster when beginning to learn a new language in adulthood.