Here are some ideas about how language learning is affected by age. You might also want to read Jean Piget's cognitive development theory.
Learning Languages as Young Children
Children are like sponges. They don't question grammar or pronunciation, they just listen and speak. A young child will have the ability to learn a language quickly and have native pronunciation. However, if the child doesn't keep up with the language, they will lose it just as quickly as they learnt it. While listening and speaking come easily, reading and writing are a bit more difficult. Teaching English to young learners has lots of tips for teaching children.
Learning Languages in School
Children in primary and secondary school get input from reading and listening. Teens tend to learn well since they still learn quickly like children and are able to study like adults. Their pronunciation might not be as good as a child who learns another language at a younger age and they might have to study pronunciation. Another challenge is that since they are fixed in their native language they might want to translate or think first in their native language.
Learning Languages as Adults
Adults have many things against them, such as being rooted in their native language and needing to understand grammar. They will want to know the "why" behind the foreign language as well. However, they are stricter with themselves and their learning. They also are usually more disciplined than younger students. They also might be highly motivated to learn a language in order to get a promotion or a better job. Due to this, they may spend lots of time studying the foreign language.
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