Earlier quoted context omitted.
I suspect adults learn languages a lot faster than kids, when we spend the same amount of time being exposed & practicing. All the polyglots say you have to be willing to mess up a lot and work to shed your fear of feeling stupid. In fact, try to seek it out because that's the primary way you get feedback & learn.
> I suspect adults learn languages a lot faster than kids, when we spend the same amount of time being exposed & practicing. In linguistics, it's pretty well accepted that kids learn languages faster than adults for neurological reasons, with considerable research support: https://en.wikipedia.org/wiki/Critical_period
There's just a lot more to learn during first language acquisition than during second language acquisition. It's possible for a child to learn faster than an adult in general, and for it to still take longer to learn a language to basic proficiency, because there's a lot more to learn the first time.
The article you linked even acknowledges that and confirms my premise from above: "Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages."
The research support on second language acquisition seems rather mixed here to me. There's certainly some evidence that kids eventually learn more, but this article presents some evidence against, and the evidence here on time to proficiency vs amount of effort isn't exactly what I'd call considerable. Most of the discussion is on whether second language learners achieve perfection, not on how long or how much effort it takes to become a basic speaker.
Very little discussion on the learning environment difference between kids and adults. Kids get full immersion and years of constant feedback. Adults usually get a few classes or books, a few hours of practice a week, and are usually speaking bilingual while learning, etc. I'd like to see studies comparing apples to apples.