Most people will tell you that the younger you are, the easier it is to learn a new language. Theorists claim that after the age of 12, children rarely acquire native-level proficiency in a second language (although there are some exceptions).
Between the ages of 2 and 4, children are at the peak of their language learning. At this receptive stage, they learn tens of words per day, at a rate quite astonishing to most adults.
Link between concept and sound is direct in children who are learning about the world and the ideas in it at the same time. These kids are not translating, which is recognised by teachers to be a helpful technique where words may not always have an equivalent in a second language.
Both at school and at home, children learn linguistic patterns and phrases through mimicry. Adults tend to learn rules and apply them to create new language, but although this logical approach is effective, there are so many exceptions to the rules it can be much slower than copying and memorising.
Personally, I suspect that there is a link between mastering the accent of a second language and learning the phonemes it uses before the mouth ‘solidifies’. The soft qualities of a child’s voice are familiar to us all, and I believe that it is at this stage, before the child develops an adult voice (by their teens) that the voice box has the adaptability to create unfamiliar sounds, and thus learn to produce the phonemes of a second language and acquire the accent.
Children with exposure to a foreign language at a young age often develop a natural sounding accent.
So the only question is why we don’t start younger. In the UK, and many other countries, languages are often introduced to the school curriculum aged 11. This is far too late for children to master a second language fluently and with ease. But providing specialised teachers tends not to be prioritised in the UK schools, in an age of attainment targets and thinly stretched budgets.