Foreign-language experience in infancy: Effects of short-term exposure and social interaction on phonetic learning
Infants are born with the innate ability to distinguish the sounds of any language, but that ability begins to diminish between 6-12 months of age as the brain settles on a native language. This study also shows that language exposure through personal interaction provides the greatest language acquisition benefits to infants, whereas exposure to foreign language recordings does not produce any significant benefits.

Language Barriers: Is it Too Late to Try?
Researchers believe that children can learn languages effortlessly until the age of 6. As children grow older, learning new languages (especially very difficult languages such as Mandarin) is much more challenging because the brain loses some of its plasticity.

Compilation of Studies on Benefits of Language Learning
Summaries of various research studies investigating the cognitive, academic, and cultural benefits of bilingualism.

Bilingual kids not slowed by second tongue
Second language learning at an early age does not hinder overall language development, and early and intensive second language learning increases a child’s ability to master the language in the long-run.