I teach English as a second language. Some people can spend literally years immersed in an English-speaking culture, taking classes, and working their butts off to improve their abilities, and still have limited English skills. Even allowing that English is a weird and difficult language to learn, learning any new language after childhood is really sodding hard. Given this, it annoys me how adult characters in Carey's books speak new tongues like a native after a few weeks of teaching and never, ever encounter difficulties in trying to communicate no matter where in the world they go. Carey stretches it with Phedre, who we were explicitly told had a lot of linguistic training as a child and an excellent memory; she absolutely breaks it with Moirin, who didn't even know other languages existed until she was in her early teens.
A couple of things: All three characters started learning new languages early on. For Imriel and Phedre it was about ten and Moirin about sixteen or so. So both Imriel and Phedre did start learning new languages during childhood and, it should be noted, are the ones that they are most fluent in. While both learn more, something easier to do for additional languages beyond the second, they're almost never described as speaking like a native. For all three to learn, communication does start out with difficultly, often takes a fair amount of time (which is, admittedly, often glossed over.
Sixteen is not childhood, especially not for these purposes. That's the whole point about complaints that high school is too late to start effective language training. However, people who've learned multiple languages when young are often much better at picking up languages later in life, so Phedre (and Imriel whom she would have given as much of the same training as possible) have some excuse.