1. Adults are faster than children at earlier stages of morphological and syntactic development. (Holding time and exposure constant)
2. Older children acquire faster than younger children.
3. Child starters outperform adult starters in the long run.
4. Attainment of native-like fluency across all domains is only possible if the language is started prior to six years of age. Learners starting not much later than age 6 can become fluent but will have an accent in phonology. Learners starting after 15 will have 'accents' in all domains.
See also DeKeyser, R. M. (2000). The robustness of critical period effects in second language acquisition. Studies in Second Language Acquisition, 22 , 499-532.
Social/Affective factors . The development of identity makes adult learners more self-conscious. Ego permeability and cultural identification make it difficult for adults to learn another language.
Input. Children receive more and better input than adults
Cognitive development. The cognitive system overrides natural language learning. The acquisition of metalinguistic skills occurs at around puberty.
Neurophysiological Loss due of natural language learning ability due to hemispheric lateralization, or changes due to mylenation of neural pathways. Recent evidence shows late acquired language to be localised in different part of the brain (Kim et al 1999).
Integrative motivation is present in learners who identify with the target culture, would like to resemble members of the target culture and who would like to participate in the target culture. It is assumed to be based in the personality of the learner.
Instrumental orientation refers to those cases where the learners are interested in learning the language for the possible benefits: professional advancement, study in the target language, business.