Get Started Early
Every infant is born with the ability to mimic the sounds of any language. But by the time a baby is about 10 months old, he begins to narrow down the range of sounds to those that he hears around him. So if you want your child to learn a second language, it's best to introduce it in the first year of life. If you are a bilingual family (or have a bilingual caregiver), have one parent (and/or caregiver) speak the second language to the baby all the time so he has a chance to become fully "immersed" in it.
If you haven't begun the second language in the first year, it's best to wait until your child is about 2-1/2 -- or until after he has undergone a "vocabulary explosion" in his first language, which generally begins at 18 to 20 months.