Their advantage is that they have almost unlimited time.
Consider how long it takes for a child to speak their first word and, then, to actually speak in well-formed sentences: Several months, even years, of complete immersion and 24/7 exposure to native speakers.
Now compare this to an adult attending a language class for the first time. Chances are, by the end of that class, they will be able to say their first words or even sentences, will understand these words' & sentences' meaning and in which contexts to apply them. Adults are orders of magnitude faster at learning new languages because they already know most of the concepts a new language's words and grammatical structures can refer to. (We all inhabit the same planet, after all.)
The only problem is: Learning all the intricacies of a language, of its grammar and vocabulary, of its melody and accent takes time and lots of continued exposure to native speakers. Adults usually don't (want to) spend that time – whether that's a conscious decision or an unconscious one.
Also, it is very not true that children do not need lessons to learn language. If anything, children receive MASSIVE amounts of explicit language training that we would never think to apply to adults. Children have songs about the alphabet and numbers. We play games with them about colors and shapes. Before the age of about 5, almost all of their toys are fundamentally designed around learning components of language. All of the books that we read them are about.
Both my 3-yr old and my 5-yr-old make what I find to be a hilariously cute error in speaking. Things that belong to them, they say are "Mines". I thought about it, and their way is more consistent. You say that toy is yours, hers, his, theirs, ours. It's only in the 1st person that we drop the -s sound at the end.
When do children gain fluency? How do we even define fluency? In the language training industry, we have the International Language Roundtable Scale (https://en.wikipedia.org/wiki/ILR_scale). The ILR Scale ranges from 0 for a raw beginner to 5 for "educated, native speaker", with people typically appending "+" to a level to blend in between levels a little. Based on the ILR scale, my 5-yr-old is a 2 and my 3-yr-old is a 1. I know full-grown adults, born and raised in America, who would probably only rate a 3+.
Children do not learn fluency without massive effort on both their and everyone else around thems behalf. And then adults complain about having to do 5 hours of homework every week and whine about not gaining fluency in Mandarin. "It's just easier for children". Yes, in a round about way, it is easier, but those reasons are purely social. Given that some adults do demonstrate the capacity to achieve fluency, yet are not living anywhere near a completely, 100% immersed life like a child does, there is clearly some natural advantage that adults have that makes up for the lack of nurture.
2. Children will learn colors and shapes just fine without explicit instruction. They do it all the time. In the pre-developed world, children didn't get taught how to speak. There were no flash cards or toys for learning numbers. They just learned by observing.
3. Letters is something else. The orthography of a language -- learning how to write it -- is a different beast from learning how to speak it. Education is necessary here. So we agree about that, for sure. No one will passively acquire how to write.
4. Children make errors in production all the time. As they learn a language they make generalizations -- generalizations that actually make sense, like "mines" -- but which are considered "wrong" by adult speakers. They'll correct themselves over time without instruction.
You might want to check out reading Steven Pinker's "The Language Instinct," it has a lot of ideas and research that might be new to you.
I like this argument, it puts it very succinctly!
On a completely different note, seeing that you're developing VR applications for learning languages: How is that coming along? Is Diplomatic Language Services already using it in production? And what improvements in learning/teaching compared to non-VR applications (and non-VR private classes) have you seen? I mean, I generally understand the appeal of VR but I hadn't heard of it in the context of language learning yet, so I'm wondering what the advantages might be since it's a purely visual thing, so a priori not necessarily more conducive to teaching a language(?)
PS: Now that I've seen you mention your alma mater on your website: What a coincidence – I once went to Shippensburg for one of my boxing fights! :) What a beautiful place!
> children never have accents, and adult non-native speakers do
This mainly due to two things:
1. Language learners usually try to map the sounds they already know (from their own mother tongue) to the new language's sounds. In fact, there are studies that suggest that, depending on their mother tongue, language learners will not even notice certain differences between sounds in the target language and their native language. (Consider how Japanese people tend to have issues distinguishing the letters R and L, or how both English and Spanish native speakers usually cannot pronounce the French/German "r" properly because that sound doesn't exist in their native language.)
This is not at all set in stone, though, because it only takes a handful of weeks of focused practice to reset your ears (and your tongue) and tune them to a new language's sounds – preferably before you learn any of the vocabulary or grammar of that language.[0] Also, the greater the spectrum of sounds you already know (the more languages you speak), the easier it will be for you to learn a new language's accent as your brain will be already attuned to listening closely to tiny differences in sound and speech melody.
Once again, consider that a child has years to learn the sounds and all these nuances.
2. Habits. Language classes almost never focus on pronunciation and speech melody in the beginning. From my POV this is a huge mistake as it means that language learners attending such classes will sooner or later get into the habit of pronouncing words of the new language using their native language sounds. That is, when they see (or think of) a word in their new language, they will no longer pause to think about how to pronounce it – they will just do it. Unfortunately, at this point it's pretty much game over as it will take a lot of work to change these habits. Then again, a lot of people also don't really care that much about having an accent.
> Another is that children do not need lessons to learn a language, and adults always do.
This is not true and I know a few people who have gone the full-immersion-zero-lesson route. It is incredibly hard, though, given the time constraints you usually have (usually a few months to a year) and you will usually progress only very slowly. Again, just consider how much time it takes a kid to learn a language by just observing and mimicking others! For adults, lessons are simply a much faster way to get started and become somewhat proficient in a language. Also, once they've taken a few lessons, they will be orders of magnitude faster at learning the rest of the language.
[0]: For an introduction to this approach of learning a language, I can recommend Gabriel Wyner's book "Fluent Forever" and, also, his pronunciation trainers and his YouTube videos on the International Phonetic Alphabet (IPA). They got me from speaking absolutely zero Spanish to being asked whether I'm a native speaker in about two months of living in Colombia. …which doesn't mean I was speaking perfect Spanish at all – but it sounded like I was! In fact, on various other occations, people told me that they could tell precisely which city I had learned Spanish in because I had become so attuned to the accent people speak there.
