Children are better than adults at learning a foreign language because their prefrontal cortex is undeveloped. This immature prefrontal cortex actually helps them acquire new information with little effort. 
Adults have a much harder time learning new languages than children due to their advanced logical brains getting in the way of the more basic intuitive forms of learning.

Adults Vs Children: Learning Ability

Learning depends in part on two distinct memory systems that incorporate different brain regions: Declarative and non-Declarative.<ul><li>Declarative memory describes memorizing concrete information (e.g., facts and events — things that can be stated or declared).</li><li>Non-declarative memory describes acquiring skills and non-conscious or implicit knowledge, reactions, and habits — things like playing the piano, fearing spiders, or improving your golf swing. </li></ul>Sometimes these two memory systems compete.Activation of one can suppress activation of the other.

Declarative vs. Non-Declarative Learning

Intentionally trying to learn specific information can get in the way of unconsciously learning patterns and intuitions. <ul><li>Growing evidence suggests that language learning depends largely on non-declarative memory.</li><li>Much of language is built on patterns and probabilities rather than precise facts and rules.</li><li>When acquiring language naturally, children pick up on these tendencies without even noticing.</li><li>Even when they make mistakes, children tend to follow the general patterns.</li></ul>

Using The Right Learning Technique

<ul>
<li>When adults try to learn a new language, they tend to rely on their prefrontal cortex (PFC).</li>
<li>Growing evidence finds that learning can improve when the PFC “turns off” and allows the more primitive, less-familiarized non-declarative memory system to take lead.</li>
<li>You can’t turn back time to become a child again, but there are ways to encourage your PFC to relax so that your non-declarative memory system can take the lead. </li>
<li>Numerous studies find that <a href="https://www.frontiersin.org/articles/10.3389/fnhum.2021.719509/full#:~:text=Mood%20and%20alertness%20improved%20after,different%20from%20the%20SOCIAL%20condition.&text=Thus%2C%20breaking%20up%20prolonged%20sitting,memory%2C%20mood%2C%20and%20alertness.">short bouts of physical activity</a> reduce prefrontal cortex activation. Listening to music or a shot of alcohol also helps.</li>
</ul>

Learning a Foreign Language: Improving Learning In Adults

It's not that you shouldn’t spend time working on your new language; you should. But instead of actively trying to focus and memorize, you’ll likely benefit from allowing yourself to experience the language. Watch TV shows, attend discussion groups, or listen to music in the language. And when you do have to remember a list of vocabulary or grammar rules, try taking a stroll first or listening to some instrumental music while you focus.

Bottom Line: To Learn More, Try Less

The basics of learning a foreign language

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Learning A Foreign Language

Despite the fact that adult brains are far more developed than children's, adults have a much harder time learning new languages.