Dear Mr. Darmanin,
you are confirming that language learning becomes more difficult the older you get and that children are learning languages more easily.
However I think that most of them especially pupils have learned e.g. English the way you are condemning it - by grammar and vocabulary.
Why shouldn't this work also with adults?
One reason why it might be more difficult for adults is the fact that for pupils it was a must to learn, whereas adults are learning without being afraid of bad marks and they do it voluntarily (and we all know how difficult it is when your inner temptation prevents you from doing what you intended).
I'm convinced that learning a foreign language without learning the grammar will only be possible when you are living in the foreign country for some time. As soon as you are able to see grammar as a great support you will certainly accept to learn it. No doubt that listening to songs and watching movies in foreign languages will help!