Three stages of early bilingualism:

(1) the child has one lexical system which includes words from both languages;

(2) the child distinguishes two different lexicons but applies the same syntactic rules to both languages;

(3) the child has two linguistic codes, differentiated both in lexicon and in syntax, but each language is exclusively associated with the person using that language.

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Bilingual children have been shown to differentiate their languages as young as two years of age, and possibly earlier. In terms of developmental milestones, research suggests that there is no outstanding difference between bilinguals and monolinguals.