As far as I know, the degree to which second-language speakers can acquire native-like competence in domains other than phonetics is somewhat debated.
Links? As far as I know it's not debated.
there are some aspects of language (I'm told that subjunctive in French and aspect in Slavic languages may be examples) that may be impossible to fully acquire for non-native speakers.
That's, ahem, bullshit. Why in the world would some features of syntax be "impossible to fully acquire"?
for all practical purposes, the claim that you have to learn a language as a child in order to become perfect (in the sense of native-like) with it is true.
For all practical purposes it is NOT true.
You may easily know more about this issue than me, because I haven't actually researched this.
That said, let's be more precise. If we're talking about mere fluency, there is, of course, no question.
But if we're talking about actually native-equivalent competence and performance, I have severe doubts that this is even regularly achieved. How many L2 speakers of English do you know who never, ever pick an unnatural choice from among the myriad of different ways in which the future can be expressed in English? This is something that is completely effortless f...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.