We lose the ability to naturally discern subtle differences in consonant and vowel sounds beyond an age, but it is totally incorrect to suggest you can't "learn" them later.

If you're simply trying to discern the sounds in a language you can become arbitrarily good at the task (as in, your ability to discern the sounds used in a language scale with time spent invested practicing it, up until well after you exceed the capacity of a typical native).

The issue is that most foreign language learners never spend that amount of time and often learn to speak before they learn to listen/understand. This causes them to permanently ingrain their incorrect pronunciation. What everyone else is left with is the illusion that complete 2nd language acquisition is impossible, but this is just wrong.

Agreed on all points. That loss of abilities that you mention is a definite challenge. That just means that a person has to work harder and smarter to learn new languages later. But, as you pointed out, it is far less likely that they can gain native fluency and pronunciation - at least in a language with sounds that are not found in one's own language.

And regarding making mistakes... I have found that the most embarrassing mistakes I've made in other languages have cemented in my mind the correct way of saying things far more efficiently than any other method. In other words, if any of you are hesitant to make a mistake in another language, fear not - let the mistakes fly and the blushing commence.

This might be generally true, but is not always true. I, for one, have a condition where I cannot filter sounds. I cannot focus on any particular sound unless it is significantly louder than all surrounding sounds. No matter how hard I try, if you speak to me as a train goes by, I will not be able to understand what you are saying if my life depended on it. I have been through many tests and have been told it is an issue with the way my brain processes sound, not my ears themselves. As such I absolutely need subtitles to understand most movies.

As a side effect, I am easily able to differentiate even the most subtle differences in language. Although I am aware that most people cannot. It has given me an edge in language learning and have learned 3 languages as an adult. 2 of which I speak at near native level fluency and am constantly mistaken as a native speaker due to my near perfect accent. Still working on the 3rd. It takes practice and time, but I can imitate the exact pronunciation unlike many other language learners.

I'm sure there's plenty of other people like me out there who can also tell the subtle nuanced differences between similar sounds, even long into their adulthood.

It's true that we stop discerning differences between sounds that don't carry meaning in our native languages, as you said, but I think it's possible to regain the ability later in life if you become very accustomed to the language and you find those differences mattering in your every day linguistic environment.

I gradually became fluent in Japanese over the 6 years I lived in Japan, and just last year I realized the way I had been pronouncing ん "n" in some contexts was wrong, but I only realized because I first noticed the way a native speaker was saying 千円 "sen-en" (the way I'd been saying it) was something more like se-yen (which maybe explains why in English we call it yen instead of 円 "en").

There were other pronunciation things I realized over time like this. So it takes a long time, but in my experience you can re-learn to make differentiation like that, and you really do start hearing things you couldn't before.