A CP was originally postulated by Lenneberg in 1967 for L1 acquisition, but considerable interest now surrounds age effects on second language acquisition (SLA). This assumption stems from ‘critical period’ (CP) ideas.

Although evidence for L2 learning ability declining with age is controversial, a common notion is that children learn L2s easily, whilst older learners rarely achieve fluency. Second language acquisition - Summary of Critical Period Research to date How children acquire native language (L1) and the relevance of this to foreign language (L2) learning has long been debated.