I haven’t had chance to watch the vid (no time at the mo as at work) so not sure what arguments are made but presumably it boils down to adult brains are different and adult learning strategies are different.
In general adult learning strategies are better though so it is certainly not impossible to learn a language well as an adult.
I think this one has been posted previously which sort-of counter-argues the point.
Interesting, @james but I think his argument is flawed towards the end. He’s right on the money when he observes that children get a huge amount of input (and praise, and feedback) which we as adults can only dream of.
He seems to be confusing fluency with being-indistinguishable-from-a-native-speaker. I have only ever known one person who matched the second description, and he was a skilled linguist who lived in Spain for a few years.
I think, given enough time, I would be able to make the p/ph distinction he talks about. I could certainly be fluent if I spent 5 years immersed in the target language!
Up to a point.
How do you mean?
Sorry I mean I agree with him up to a point! I agree with you.
Having now watched the video it does not seem that many points are made.
We have the, I think uncontentious, claims that its a little bit of a myth that children “easily” pick up language, or that they do it quickly, and that there does seem to be a period where humans pick up and learn to generate sounds in a language which diminishes - presumably explaining why the French famously have difficulty with the English “th” sound and I can’t do a proper French “r”.
He goes through a couple of (discredited) linguistics theories, and points out that adults learn in different ways to children (also explored better by Gabriel Wyner).
However my frustration is that, unlike Wyner, he offers no insight into how to improve our ability to improve our ability to learn a language, and he does not actually  answer the headline question.
I think that we differ in our ability to absorb language as an adult - and we clearly differ in intellect and how well we structure our learning but, as Wyner says immersion is an excellent tool as are the online learning tools and making associations between the sounds and actual objects and experiences. I think it’s actually a mistake, though one most adults (including me) are constrained to make, to learn vocabulary in one language in terms of another - this is the real difference, and weakness of, adult language learning compared with how children do it.
Wyner, despite his early experience, clearly has a gift for languages but I don’t think he is so far up the curve that what he found useful is not generally applicable.
I know my own ability to speak French is still about a million miles from where I want it to be (despite trying to learn the bloody language for 40 years) but I also know that in the course of visiting the Brocante and spending half an hour ordering a new table for the house I learnt three new words1 just listening to the sounds and thinking about how they fitted into the sentence - so if I do ever fulfil my desire to more to France permanently I’m hoping a mixture of immersion and hard work will pay off.
1] sensiblement for “pretty much the same”, fendre for to split/chop and I forget the other - but as Meatloaf said two out of three ain’t bad.
For me, immersion was definitely the best. I had done 2 years of French at School, but that was in the dim and distant past.
When we first arrived, I would sit at a neighbour’s table, concentrating hard as everyone babbled around me - only picking up the very occasional word and carefully translating “it” in my head. Over time, I could pick out more words, but I took so long to translate that I was way behind in the actual thread… just smiling at folk as and when the occasion seemed to demand it.
In those days, two words strung together and lots of gestures was quite an achievement… if it meant I could make myself understood. Not having other English around (apart from OH) meant that I simply had to get down to learning the language.
I found myself muttering translations of every sign and notice I came across - drove OH nuts, but it helped me. Often, I would ask a shop assistant to repeat “whatever” and to explain what that word/phrase meant… (I carried a small dictionary everywhere - very useful but often lacking… 
)
The time finally came when the sense of a conversation would simply wash over me… without my brain translating word for word… phew.
When I was learning Arabic at first, listening to the radio was pretty meaningless, it was just an uninterrupted stream of sound (my undergraduate course wasn’t primarily about learning to speak* but all about reading and writing the language) but learning the rules and vocab and more vocab then more vocab finally paid off and the torrent resolved itself into recognisable units. Arabic also contains consonants we simply don’t have in W European languages so speaking is tricky until you get them right.
I have French friends who simply don’t hear the difference between eg a long and a short vowel (sheep is long, ship is short) and I simply cannot imagine being in that situation.
*we were told by the Professor of Arabic on our first day in the faculty that if we wanted to learn Arabic just so we could speak to Arabs we’d be better off going to Berlitz.
Plus, our hearing is not so acute.
Jim still thinks that it it is another entirely different verb if there is a l’ in front of it!
“I have French friends who simply don’t hear the difference between eg a long and a short vowel (sheep is long, ship is short) and I simply cannot imagine being in that situation.”
My Greek friend is the same, though in every other respect is fluent and has no difficulty in communicating complex ideas (she is an engineer, did a PhD in the UK, works there).
And she still struggles to say “squirrel” which causes me (who speaks - well, one iota of Greek) endless amusement.
@anon88169868 that’s rubbish, Paul: we’re bursting to know the third word 
I am learning that, for me, the most important thing is to “remember” that I’m good at languages. I already have native-speaker fluency in one, so why shouldn’t I approach that in a second?
We don’t have to have “moktor” moments necessarily: indeed, your experience in the brocante shows that you need little more than a lively interest in a subject and a willingness to ask questions and, perhaps, to look stupid.
More power to your (and our) elbows!
Up to the age of around 6-7 children are far more receptive to learning another language, particularly by immersion (the way they learn their mother tongue).
Once they have a second language they become language sensitive.
Children older than that and adults have fixed constructs and accents from their mother tongue which makes it more difficult to absorb a second language.
Formal teaching mathods tend to concentrate on grammer and spelling rather than assimililation through use and need as happens with immersion.
I had a poor O’level in French but immersion when we arrived here 20 odd years ago helped me to be able to communicate verbally on the subjects I engaged in (property renovation, football, politic…).
I can now read French pretty well, even in specialist subjects but I write French poorly. I can communicate effectively in spoken French provided that I haven’t had a break from using it for several days/weeks.
Our son, who was 4 when we arrived here, has none of these problems and has picked up other languages along the way.
Children of prople who move to another country are far more exposed to the host language all the time, also they are actively expected to learn, people correct them etc - more complicated for adults I suspect.
I have learnt various languages at various stages of my life but I grew up speaking & literate in 3 so I don’t count.
Veronique,
that mirrors our experience.