How do children become bilingual? Is there a sensitive period for second-language development?

What will be an ideal response?

Children can become bilingual in two ways: (1) by acquiring both languages at the same time in early childhood or (2) by learning a second language after acquiring the first. Children of bilingual parents who teach them both languages in infancy and early childhood separate the language systems early on and attain early language milestones according to a typical timetable. When preschool and school-age children from immigrant families acquire a second language after they already speak the language of their cultural heritage, the time required to master the second language to the level of native-speaking agemates varies greatly, from 1 to 5 or more years. Influential factors include child motivation, knowledge of the first language (which supports mastery of the second), and quality of communication and of literacy experiences in both languages at home and at school.
As with first-language development, a sensitive period for second-language development exists. Mastery must begin sometime in childhood for most second-language learners to attain full proficiency. But a precise age cutoff for a decline in second-language learning has not been established. Rather, a continuous age-related decrease from childhood to adulthood occurs.