The evidence points to the contrary, you acquire a language as an adult faster than a child can. This is because you have more complex mental faculties available, and can have complex topics explained as analogies to a language you know.
As others have pointed out, ESL learners ask questions about English articles all the time, and it helps them understand how the language works faster than if they had to brute-force the patterns.
There is no "natural" way to learn, there's just ways that work and ways that don't. The fastest way to learn a language is by having comprehensible input, and more input becomes comprehensible with directed, structured study.
You may ramp up adult-level conversational ability faster, but it takes much longer to reach any state of native-like fluency.
For example, I lived and studied in Japan, and while I probably jumped from zero to ~middle-school in terms of basic communication ability in about a year with daily classwork and total immersion, I was nowhere near the average toddler in terms of basic fluency after that time. Even now, I can make myself understood in most adult-level contexts where a toddler would be helpless, but nobody would ever confuse me for a native toddler.
It's not just accent, either -- little kids still routinely teach me vocabulary and grammar.
If you had three years of paid vacation where you had to use the language exclusively for everything, your skills would probably improve to toddler-level or beyond. Though admittedly you already know many things that they have to learn at the same time.
Having to communicate with your family (and probably close friends/significant other/etc.) exclusively in the language probably makes a big difference as well.
I think a lot of fluency is having someone carry you around 24/7 and talk to you like a baby for a few years. Much of the rest is just having patient people around you for another 3-4 years who will listen to you babble like an idiot and correct you when you're wrong.
I agree that if I could do nothing but spend 3 years in total immersion, I'd probably accumulate an adult-level vocabulary (again, the ramp-up is steep), but I still wouldn't sound like a native at that point, whereas a toddler would sound like a native...but have a toddler vocabulary.
Hmmm I think if you observe children my personal conclusion is that in practice (over theory) thought and language are intractable and as we mature, expression becomes more acute and so language acquisition is faster because we no longer need to learn how to think.
A 6 year old needs to learn $LANGUAGE and science simultaneously
Indeed, that's why adults are still favored in language learning, with the exception of phonology (which does make sense, retraining all those fine movements truly is harder than starting with the right movements to begin with) - https://sites.psu.edu/bilingualismmatters/children-vs-adults...
Adults win in acquisition speed because we already have a template for learning a language.
As others have pointed out, ESL learners ask questions about English articles all the time, and it helps them understand how the language works faster than if they had to brute-force the patterns.
There is no "natural" way to learn, there's just ways that work and ways that don't. The fastest way to learn a language is by having comprehensible input, and more input becomes comprehensible with directed, structured study.