About the innate ability of children to languages
Researchers have established that all languages of the world share a common grammar, even if they differ significantly. Children have an innate understanding of such basic grammatical structures that underlie all languages. This is sometimes called the "language instinct".
It is strongly tied to spoken language. Therefore, a child acquires it within a few years, while the process of learning spelling takes much longer. This happens because information perceived by the organs of hearing is directed straight to the appropriate areas of the brain. That's why foreign languages are easier to learn through communication.
What's interesting is that abstract perception of words is also innate. Children understand that the word "cup" refers not to the specific cup they're being shown, but to all cups of different shapes and sizes in general.
Sometimes it happens that peoples speaking one language mix with others speaking a different one. To interact together somehow, they create a mixture of all these languages. Initially, it's terribly inconvenient, crooked and awkward. But in the next generation everything changes - children take over, communicating among themselves and creating a full-fledged language.
A similar situation occurs in communication among deaf children if they're not particularly worked with - it proceeds with difficulty. But at the same time, there's rapid language development among the youngest ones, who form their own sign language that's not inferior to any other language.
From this it's clear that children's ability to acquire languages is lost with age. There are such examples in the animal world too. For instance, a finch can learn the song of its species only if it hears it at a certain age.
Sometimes very sad cases confirm this too. One girl from Los Angeles was kept in a small room for 13 years with practically no human contact. As a result, she knew only the phrases "Stop it" and "No more". After her release, she quickly expanded her vocabulary, but was never able to form sentences.
Sometimes you can hear people say that we all learned language in childhood, so we can now learn foreign languages without problems. But back then our instinct helped us, which was lost with age.