I'm not quite sure what you're trying to say by "children never have accents", but children do have accents, both the accent of whatever locality in which they live, as well as their own accents from not learning the language precisely. My 3-yr-ld son pronounces all words end in an "-ar-" sound of some kind as "-aiee-". He wanted to play "cards" and it came out "cai-eeds". There are other, similar foibles in the language of children that I think could easily be called "children have a unique accent".

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.

Those things are fairly easy to explain and they also aren't god-given facts.

> 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.