An old prejudice says that multilingualism demand too much for children and no language is learn correctly. But some researches prove that it is easier for a child to learn a second language. Babies hear their mother's voice even in her belly. When they are born, they can understand and distinguish sounds of all the languages in the world with clarity and precision. The first seven years of the child are the best to learn foreign languages. The brain of a child until that age is more responsive to language differences. Until the age of three the different languages are ''stored'' in one brain region that is why the children's brains work very efficiently.
Let us compare the languages learning abilities of a child and an adult. The people who have learned a second language during the period of childhood have a better chance to speak that language with pronunciation of someone who's that his is mother tongue unlike adults who have a better chance to speak with an incorrect accent when they speak a second language. It seems that the age does not affect the skills of learning the vocabulary of a second language. Regarding grammar there is no unanimous position. An interesting fact states that if the second language learns is close to English, adults may even be more competent than children in the mastery of English grammar. Also the way that children learn a language is different than the way the adults learn it. Children naturally associate movement and action, image and sound, objet and noun. It's the same for the sounds which are perceived differently by adults and children. Indeed children are better to detect the different sounds of a foreign language.
As we've seen children learn more easily a second language than the adults. It is logical to conclude that it is more difficult when they get older. Actually when the child becomes accustomed to his mother tongue he loses his capacity to hear and distinguish the subtlety of the sounds of a foreign language.