"More systematic evidence comes from the psychologist Elissa Newport and her colleagues. They tested Korean - and Chinese - born students and faculty at the University of Illinois who had spent at least ten years in the United States. The immigrants were given a list of 276 simple English sentences, half of them containing some grammatical error like The farmer bought two pig or The little boy is speak to a policeman. The immigrants who came to the United States between the ages of three and seven performed identically to American-born students. Those who arrived between the ages of eight and fifteen did increasingly worse the later they arrived, and those who arrived between seventeen and thirty-nine did the worst of all, and showed huge variability unrelated to their age of arrival." Steven Pinker, The Language Instinct.
What seems to be happening in Taiwan is that the drive to English proficiency is running into the buzzsaw of Taiwan identity politics, and the drive to strengthen indigenous languages (ben tu yu yan). It looks as if there will be three strata of language on the island: English, for communication with the world; Mandarin, as an island-wide lingua franca; and indigenous languages - Taiwanese, Hakka, and aboriginal languages. A foreigner needs to be a bit circumspect about coming down too hard on these kinds of identity issues, which the Taiwanese have to work out on their own. What sets alarms off for me, though, is when a political decision is made (i.e. Taiwanese before English), and then, extrapolating back, academic theories are constructed to conform to that political need. That is the process described in the L.A. Reader article on Krashen below. Politically inconvenient it may be, but ten and thirteen year olds do not acquire languages (first or second languages) faster than kindergarten children.