sentences:  However, the temporally defined critical period does not apply in the same manner to every aspect of a language and it differs for the phonetics, lexical and syntactic levels of a language, though studies have yet to conclude the exact timing for each individual level.Studies on monolingual children have shown that the time before an infant turns one year of age, is an important window for phonetic learning; between 18 months to 36 months of age is an important period for syntactic learning; and vocabulary acquisition grows exponentially at 18 months of age.Behaviourists believe that social environments play a vital role in language learning.