Anyone who tries to tell you otherwise is selling bunk theory.
The formal theory is called the critical age hypothesis by Eric Lenneberg (1921–1975), and yes it is no longer canon. All research points in a slightly different and perhaps noncontradictory direction. Adults can learn language like children, just that they don’t and probably will not. Not a satisfying answer? Sorry.
As it stands, the most difficult part of languages to learn are colours. Not kidding, colours are actually really really hard to learn in a foreign language. For example English only has green and blue, but most other languages include a word that describes both as the same colour: grue. Other languages, like Japanese, include all three: blue, green, and grue. Defining word semantic boundaries is subtle and nearly impossible to translate.
Aside from colours, the most debated aspect of learning a language is grammar. Some linguists still claim that perfect grammar cannot be learned outside of the critical age. This would be an interesting claim if it weren’t that nobody learns grammar until middle school. So, counter to the claims of this theory, most native speakers learn grammar outside of the critical age.
However, this leaves us with a different and difficult question: why do second language learners struggle so much? Similarly, why do bilingual children do so well in both languages? My personal theory here is that adults learn to translate but children learn to speak.
Throughout my personal foreign language studies I have spoken with a number of students and teachers who describe this situation exactly. First I tried learning to translate with Spanish. To this day I can’t say that I speak a single word of Spanish. I spent roughly eight years, some of them in the critical language period, studying Spanish in a classroom setting. Then in the last two years of high school I started learning Japanese just by listening to it. By measure of the Japanese Language Proficiency Test, my listening score has been without flaw for tests up to the N2 level.
So, how did it feel learning two foreign languages? First, for Spanish, I learned a mapping of translated English words. Dog is perro. Cheese is queso. Run is corro. I rarely, if ever, heard full sentences of spoken Spanish in the classroom. Compare that to the roughly 3000 hours of listening to Japanese. It should be obvious what the difference is here. Adults don’t lack in learning effort. They lack in learning quality.
Exposure is key. Making associations, between what you hear and what you see, are the fundamental building block of native language. Without this experience, nobody, however intelligent, will learn the language. Age is irrelevant. Coddling is everything.