Between birth and adolescence the brain is hard-wired to acquire language naturally. As a child approaches puberty, the nature of language learning and storage changes, becoming less flexible:

Ease of learning a second language diminishes with age.

• Learning a second language at a young age is cognitively as easy as learning a first language.
  
  
• A child’s brain processes multiple languages in parallel paths, building a second language system alongside the first.
  
  
• A young learner can access a second language separately, without having to translate, or go through the native language as a path.
  
  
• An older learner (after puberty) stores new languages in a separate area of the brain, requiring translation and explicit grammar training to learn.
  
  
• The diminishing plasticity of the brain makes early learning optimal.

Taking advantage of this window of opportunity by exposing your child to a second language young allows a child to optimize his or her learning potential, helping to shape the brain at its most flexible stage. The more stimulation language centers in the brain receive during this critical window, the more neural connections develop, enhancing not only long term language skills, but overall intelligence development.

According to the Wall Street Journal, "learning a second language later in life is fundamentally different from learning it early in life... Toddlers who learn a second language along with their native tongue store this capability in a single sector in the brain... but if the second language is acquired later -- say, in a French class in high school -- the brain designates a separate area for processing it... those who learn two languages at a young age retain an [unique] ability to speak both as if each was their native tongue."