My personal experience actually maps to this exactly and, when I’ve encountered it, it’s been a far more predictable rule than I would have thought. This is anecdotal I know, and I’ve even looked up the science on this and it seems like isn’t as black and white as my experience would indicate.

But by now I’ve actually asked this of prob 20-30 people. All of them who came to the US before 12 (or attended American international schools overseas) had no accent. And all of them except two who had come here at 14 or later had at least a hint of an accent. There are the few like the one Hungarian I met who had no English other than spending 3 months in the US and whose English was so spot on I actually thought he was American.

In my experience it does seem that there’s something about the brain’s plasticity that changes around 13ish. For example, I started programming young and also had took physics early at my local college and seem to internalize those much better than, for example, the follow-on physics course I took later on.

But if anyone knows the science better feel free to correct me! A neuroscientist I am not…