You need a new excuse

for not concentrating on learning Mandarin asap... (knowing it already is probably the best one).  This seems to be another area where apparent plausibility -- an intuitive 'model' of pliable, fresh young brains -- combined with difficult to control experiments, and perhaps some wishful thinking led apparently to unsubstantiated common knowledge...

Age no excuse for failing to learn a new language - life - 22 July 2011 - New Scientist: "Age no excuse for failing to learn a new language

22 July 2011 by Catherine de Lange
Magazine issue 2822. Subscribe and save
IT'S never too late to learn another language. Surprisingly, under controlled conditions adults turn out to be better than children at acquiring a new language skill.

It is widely believed that children younger than 7 are good at picking up new languages because their brains rewire themselves more easily, and because they use what is called procedural, or implicit, memory to learn - meaning they pick up a new language without giving it conscious thought. Adults are thought to rely on explicit memory, whereby they actively learn the rules of a language."