I notice the biggest difference between me and a child is that children don't care about messing up. Meanwhile I often have this worthless apprehension of "better not practice Spanish on these Spanish speakers, I might mess up and feel stupid."
I tried to learn Spanish like I learned how to program. Now I'm living proof that you can become good enough to read books in Spanish yet amaze people with your inability understand spoken Spanish.
Two quick hacks to try.
1) Stop reading books in Spanish silently. Read them out loud. This will force your brain to create neural pathways between understanding, comprehending, and speaking.
2) Put in your earphones and walk down the street. Play something that is like a Spanish conversation podcast and try to mimic what they are saying. Doing it in public will help conquer the 'ego' that gets in the way of making mistakes in public and eventually with other people.
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've anecdotally heard stories of "gringos" picking up conversational Spanish just from working in the back of a restaurant with a Spanish-only staff.)
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.
I'd postulate it's probably easier for trained adults to learn new programming languages as they're more familiar with structured syntax and the semantics involved, and are more likely to have exposure/experience with a similar language. Also, concepts from one programming language carry over to another, even if they're expressed differently.
I don't have any studies to back this up; it's just my gut-feel.
When I speak (and learn) Japanese, my conscious understanding of Japanese grammar simply makes it easier. The same when I speak French. I don't unlearn anything; I learn different patterns, and I simply switch to them.
In fact, it's what you suggested about programming languages, but for spoken languages. In my experience, adults can learn foreign languages far faster than children can. Maybe it appears that children are better at it because they are completely immersed in the language they need to learn and have no other means of communication. If I spent 16 hours a day immersed in a foreign language, with people trying to communicate with me and no other way of communicating back, I reckon I'd pick it up pretty swiftly.
But think about how much faster you learned JS than the time it took to learn C (or whatever your first programming language was.)
Don't forget to account for the vast amount of learned information you get to leverage from your first language, even if you have to unlearn a few things or learn a few new things.
Even though JS is very different from C, learning JS was much, much faster for me than learning C was, because I already knew a whole lot about the way all languages work just from learning C.
I'm learning Icelandic right now, and it only took a few weeks of 30 minutes a day to cover some basic conversational skills, gather maybe 100 or so words of vocabulary, and get an introduction to the basic speech patterns & conjugation. It'll take a lot longer to be fluent, but children take years to get that far, because they're starting from scratch and don't have connections to build on. I see a ton of connections to German and English that kids wouldn't.
It's also hard to compare programming to high school Spanish, since it's easy for us to spend 10 hours a day programming, but in high school we topped out at like 3 hours a week of Spanish exposure, and most of that wasn't practicing very hard.
I moved to Mexico as a child (during my first grade year), and learned enough Spanish to go to school, play with the kids, etc. I feel like I'm picking up languages faster now than I did then.
and here: is Chris Lonsdale saying adults can learn useful amounts of spoken Chinese in 6 months, with immersion and being able to watch mouths closely and see how speakers move their moouths to pronounce things (IIRC): https://www.youtube.com/watch?v=d0yGdNEWdn0
and there's a bunch more; https://www.youtube.com/watch?v=-WLHr1_EVtQ and so on
We can't really do the same with languages, but it's interesting! I definitely feel how I tense up if I have to practice openly.
It's difficult to be in the open like that - but it's a lesson in humility.
It has given me a great appreciation of my children who are also in the class (at higher grades), and been a fascinating lesson in learning and development. It's helped me in many ways - I'd recommend it.
1. Don't buy a 2 or 3 octave keyboard like I did.
2. Synthesia is great. Guitar hero for a midi keyboard.
3. musictheory.net is a great interactive way to learn some basics.
My learning is mainly geared towards producing for games, so I thought I could get away with a 25 key keyboard, but I regret it a lot.
A physiological difference you wouldn't have noticed are that children will be making more neural connections than adults at that stage of their development.