I think it’s commonly accepted that the younger you start learning a new language, the easier it is to become fluent. The human brain just seems to have more plasticity at an early age, and many studies seem to give credence to that idea. But it’s not to say that taking on a new foreign language in your 40s, 50s, or even later is too late – depending on how focused you are and the time spent (plus your linguistic abilities), you can still make great progress and learn to communicate in that language to some degree.
One area that is particularly difficult to master completely is pronunciation. Depending on where you live and work, there’s a good chance that around you are some people who are quite fluent in English but have an accent because they started learning English a little later in life. For that reason I don’t expect to ever be able to polish my Japanese to the level that I am indistinguishable from a foreigner. Honestly if I can master grammar completely and be able to fluently speak about almost anything on my mind, I’ll consider my foreign language study a success.
In my own life experiences I look for events that relate to learning (and sometimes unlearning) of foreign language accents, and sometimes these can point to interesting things about how the mind learns languages.
One such anecdote is when I was drinking alcohol with some friends that spoke Japanese, and later I was told that my Japanese pronunciation got significantly worse (had more of an ‘accent’) when I was inebriated. Of course my English was likely slurred to some extend, but the interesting thing is that my Japanese pronunciation was not only slurred, but it seemed to devolve to sounding more like English.
Another story that stands out in my memory is when a workmate told me how a relative had gotten Alzheimer’s, and around that time a thick accent returned to his English speech which had long disappeared several decades back.
What’s common between these two cases is that the way we learn and retain things is different between an adult an a child. The way I think of it is that our native language we’ve learned as a child is more ingrained at a lower level (In computer terms, this seems to parallel to the concept of ‘hardware’ or possibly ‘operating system’) and things we learn later are done at a much higher level. Foreign language pronunciation learned on a high level can sound quite natural, but it takes more effort and processing time, both consciously and subconsciously. For that same reason, I can listen to English in the background while attending to some other task and understand the general gist of what is being said, but with Japanese I have to make a much greater effort and cannot effectively multitask listening.
If you have any interesting or educational stories about foreign language accents please let me know!