From the module "Second Language Methodology and Language Proficiency" from course CIL 751.
ARTICLE: Language Teaching Methodology

Second language learning by adults should not be modeled on first language learning by children. Adults’ brains have already passed the critical age for natural language acquisition. Research done on abused children, who were isolated and prevented from acquiring language until after puberty, suggests that this critical age exists because these children were never able to acquire native-like fluency. Research has also shown that literate adults learn a second language, in part, by comparing it to the grammar and vocabulary they already command in their L1. Children do not have a prior grammar to base their L1 learning on. In addition, adults encounter language in very different ways than children do. Children do not need to be able to communicate clearly to have their needs met, but adults do have a pressing need to do so. Also, adults use a different register to speak with children than they do even with non-native English speaking adults. Therefore, attempting to envelope an adult language learner in the same language context as children is impractical and would be ineffective even if it were accomplished.