The way your brain is "organized" determines in part how good you will be at learning a second language. However, it is still unclear how other factors might affect your ability to learn a second language. Things like motivation and dedication to learning can have an impact as well. Nobody should be discouraged from learning a new language in adulthood. If you have the determination and work ethic to become fluent in another language you can definitively do it. While some people have a natural ability to learn a second language that makes it easier to achieve fluency, anyone can make up for not being a "natural" at it by working hard. Just like other activities such as sports, a musical instrument or even being good at cooking or at your job, it takes focus and attention to detail to master anything. Below is a portion of an article by Brianna Yamasaki from the University of Washington on this topic:
"A new brain measure for language aptitude. When we correlated our measures with learning rate, we found that patterns of brain activity that have been linked to linguistic processes predicted how easily people could learn a second language.
Patterns of activity over the right side of the brain predicted upwards of 60 percent of the differences in second language learning across individuals. This finding is consistent with previous research showing that the right half of the brain is more frequently used with a second language.
Our results suggest that the majority of the language learning differences between participants could be explained by the way their brain was organized before they even started learning.
Implications for learning a new languageDoes this mean that if you, like me, don’t have a “quick second language learning” brain you should forget about learning a second language?
Language learning can depend on many factors.
First, it is important to remember that 40 percent of the difference in language learning rate still remains unexplained. Some of this is certainly related to factors like attention and motivation, which are known to be reliable predictors of learning in general, and of second language learning in particular.
Second, we know that people can change their resting-state brain activity. So training may help to shape the brain into a state in which it is more ready to learn. This could be an exciting future research direction.
Second, language learning in adulthood is difficult, but the benefits are large for those who, like myself, are motivated by the desire to communicate with others who do not speak their native tongue."
You can find the full article here: theconversation.com/why-its-hard-for-adults-to-learn-a-second-language-61477