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.
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 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.
A 6 year old needs to learn $LANGUAGE and science simultaneously
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.