My parents know all the restaurant related stuff. Food, money, business, smalltalk. Unsurprisingly.
They have an accent as well. I guess that's because at some point people stop being corrected on the exact sound they are making.
Now I happen to be taking mandarin lessons at the moment. My teacher is very particular about the exact shape your mouth and tongue should be making. She's the first language teacher I've had who's done that.
Likewise with other language learning issues. When you're an adult, nobody corrects your grammar once it's good enough to be understood. Or worse, they just use you to practice English.
Also there are great examples of late learners. The three Danish Princesses, Alexandra, Mary and Marie, from Australia, HK, and France. They all went on intensive courses in Danish which is a weird, weird language. They're pretty good at it.
Also I came across a French guy who sang in perfect Vietnamese on TV once. Don't recall the name.
Aside from that I've heard stories about diplomats who've gotten quite good.