Answer:
Learning something new, such as a foreign language, might be a challenge for a lot of people, especially adults. Besides their fear to make mistakes and embarrass themselves in public, adult students tend to transfer the knowledge of their own native language to the target language. A result of this is a foreign accent, in which students use the sounds of their mother tongue in the new language. That is why it is said that it is difficult for adult students of a foreign language to achieve high levels of proficiency.