Research indicates that there is a 'window of opportunity' for learning a second language. That is, the capacity to distinguish and make sounds (phonemes) diminishes with age. Older children and adults can still learn a second language, but it tends to take longer. Also those who learn at a later age are less likely to speak like a native (i.e., without an accent; Golinkoff & Hirsch-Pasek, 2000).

Some people worry that starting kids on a second language will interfere with their ability to learn English language skills, however the opposite appears to be true. There is recent research about the dual language approach to learning language being effective, as well as immersion (King & Mackey, 2007).

What evidence is there for how learning a second language learning can benefit children’s development (cognitive, social, academic, etc)? 

Responding to this question "counts" as a discussion response. 

References
Golinkoff, R. & Hirsh-Pasek, K. (2000). How babies talk: The Magic and Mystery of Language in the First Three Years of Life. New York, New York: Plume.

King, K. & Mackey, A. (2007). The bilingual edge: Why, when and how to teach your child a second language. Collins Living.