Adults can think in their own language, quickly form social circles within the same language, and most importantly, identify themselves as "foreigners." Children are not "grown-up" enough to do these. Not to mention the society's different expectation to the ability to comprehend towards adult and child. 

–Notes retrieved from my passionate sidenote on "Adult Language Learning"

Cited text: 

Cases are always mentioned of adults who live for years in a foreign country, surrounded by a language which they try to learn with only limited success. But within a few months their children, in the same circumstances, may master the new language so well that they are called upon to help their parents as translators. 

–G. Hudson, Essential Introductory Linguistics, 1999

P.S. English is terrible on making sidenotes. You cannot even finish a word in a line, while the same space is enough for five Chinese characters.