Have you tried learning Japanese / English after learning the other? I studied Japanese and learned how to pronounce the /r/ in Japanese correctly.

For some people, the difficulty is less in production, and more in interpretation for someone who is native Japanese speaking and later learned English.

Chiming in with more context, my PhD was in neuroscience and I worked in a language lab. As others have stated, there is a critical window for learning a language. The biology behind it is fascinating.

As early as about 9 months of age, your brain begins to decide what speech sounds are important to you. For example, in Japanese the difference between /r/ and /l/ sounds doesn’t matter, but in English it does. Before 9 months, most babies can tell the difference between the two sounds, but babies living in Japanese-speaking environments (without any English) LOSE this ability after 9ish months!

Language is more than just speech sounds, though. Imagine all these nuances of language - there are critical moments where your brain just decides to accept or reject them, and it’s coded somewhere in your DNA.

It proved there were benefits, read the article.

Very few things are proved definitively in science. You test a hypothesis with statistics, which always carries a margin of error. Usually, it’s 5% - the probability that your data randomly supports your hypothesis, even though there’s no true relationship.

Personally, I prefer when journalists coach their language to avoid overstating the truth.