I feel like we're so deep in the thread - where after people have time and time again disproven the original comments message about how being 12 years old is some magic cut-off point to language learning - there are now people who just keep adding different requirements, leading this conversation nowhere and making the new point unfalsifiable and - by extension - unscientific; whereas the original comment had an allure of having a modicum of scientific rigor behind it (which, in the end, was disproven)

I think that the original comment has some anecdotal truth - because the vast majority of people do not spend the effort required to speak their second/third languages without an accent. You have to do silly exercises and stuff for very little gain in the end. You're better off learning a 3rd language.

Professional Actors do it all the time, they spend a lot of time practicing (and I assume money) being trained by Speech Therapists to different perfectly.