There are two main types of language acquisition when a second language is involved: simultaneous bilingualism and sequential bilingualism.
Simultaneous bilingualism refers to when two languages are acquired at the same time, where both languages are introduced before three years of age. A real-world example of simultaneous bilingualism would be when each parent speaks a different language to the child from birth onward. This method allows the child to be exposed to both languages in the home and to learn both languages before the age of three.
Sequential bilingualism refers to when a second language is acquired after the first language has already been established, where the second language is introduced after the age of three. A real-world example of sequential bilingualism would be where one language is spoken in the home and the second language is introduced once the child attends school.
Babies and young children are naturally wired to learn languages. Research has shown there is a critical period for language learning. The sensitive period for the acquisition of phonology (sounds of a language) ends at 12 months of age (Ruben, 1999). By ten months of age, children already begin to prune sounds not present in the language they are exposed to (Werker et al., 1981). The sensitive period for the acquisition of syntax (the allowable pattern for combining morphemes and words into larger units) ends at four years of age and the sentence period of semantics (the meaning of words) ends between 15-16 years of age (Neville et al., 1992).
Thus, it is best to start early and expose the child to both languages during this sensitive period. Simultaneous bilingualism is generally preferred to sequential bilingualism in order to take advantage of babies’ and young children’s innate language-learning abilities and their high degree of neuroplasticity (the forming and reforming of neural pathways). For ways to teach your child a second language, check out my articles “Fostering Bilingualism in the Home” and “Methods for Bilingual Parents to Encourage Bilingualism in the Home.”
References:
Ruben, Robert J. “A Time Frame of Critical/Sensitive Periods of Language Development.” Indian Journal of Otolaryngology and Head & Neck Surgery, vol. 51, no. 3, 1999, pp. 85–89., https://doi.org/10.1007/bf02996542.
Werker, Janet F., et al. “Developmental Aspects of Cross-Language Speech Perception.” Child Development, vol. 52, no. 1, 1981, p. 349., https://doi.org/10.2307/1129249.
Neville, Helen J., et al. “Fractionating Language: Different Neural Subsystems with Different Sensitive Periods.” Cerebral Cortex, vol. 2, no. 3, 1992, pp. 244–258., https://doi.org/10.1093/cercor/2.3.244.
