The outcome of first language acquisition is success: normal children acquire the grammar of the ambient language. Adult second language acquisition, on the other hand, results in varying degrees of success. Failure to acquire the target language is typical. (Birdsong, 1992: 706)

It is true that many adults learn to communicate effectively using an L2, and some few appear to have extensive if not perfect knowledge of the grammar of the L2. Nevertheless, the overwhelming majority are not able to achieve anything like the same level of mastery as that achieved by every normal child. (Schachter, 1996a: 160)

It is much more difficult to learn a second language in adulthood than a first language in childhood. Most adults never master a foreign language, especially the phonology – hence the ubiquitous foreign accent. Their development often ‘fossilizes’ into permanent error patterns that no teaching or correction can undo. (Schwartz, 1997)

It has been widely observed that children from immigrant families eventually speak the language of their new community with native-like fluency, but their parents rarely achieve such high levels of mastery of the spoken language . . . Many adult second language learners become capable of communicating very successfully in the language but, for most, differences of accent, word choice, or grammatical features distinguish them from native speakers and from second language speakers who began learning the language while they were very young. (Lightbown & Spada, 1999: 60)