Quattrocento, I do work in the Modern Languages department and a lot of work is currently being done there about language acquisition in adults. Not my area of expertise, but I think it is fair to say that a far more nuanced picture is emerging than the old "window shuts after 11" take.
Which is just as well- a fair bit of our own language teaching at the department would be wasted if the students were too old to actually learn anything .
But note that I am not saying that it isn't harder, or that the learning experience isn't different: what my colleagues do seem to be finding is that you can get a perfectly worthwhile outcome by using a different route.
And I don't think my quoting the Swedish example is just my own subjective experience. It is a well known fact that the Swedish economy depends on trade and industry, that you can't do trade or conduct business without being able to communicate, that for the Swedes this has to be in the language of the other partner for obvious reasons, that a lot of it is conducted in German, and that noone in Sweden is offered tuition in German before secondary school
(many only have the opportunity to take it up in college). As for English, there is no teaching of English in Swedish schools until age 10.
However, from the Swedish pov it doesn't matter if language learning under these conditions is harder, or impossible, or whatever: people have to do it to get a job, so they do do it.
For the record (now dipping into the anecdotal), I don't remember anyone telling us that language learning wasn't hard. They just told us it was necessary .