Have you ever noticed how learning a foreign language is much more difficult as an adolescent/adult than it is when you were younger? Recent scientific research has verified this. The brain is apparently most equipped to pick up a new language before you turn 17. The conclusive reason for why this happens has not been found out yet. However, it is thought that it might be due to decline in brain plasticity i.e. the ability of the brain to rewire itself. Socio-cultural factors may also have a bearing.
Ideally, you should start learning a new language by the age of 10 to gain easy fluency in it. Any older and your language learning abilities will be impaired. Most scientists agree that age 3-4 is the best time to introduce a child to a second language as this is when they have a natural ability to learn. Skeptics may say that a child is too young for second language acquisition during this time, but research says otherwise. It has been found that bilingual children who learn both their primary and secondary languages parallelly are more fluent at both than children who learn their mother-tongue before beginning on another language.