I only have this anecdote. My wife, two kids (3 & 5 years old) and I have been taking Mandarin classes for the past six months. My wife & I pick up the concepts, grammar/patterns and vocabulary about three times faster than our kids (we are also forced to work harder at it because we're constantly reinforcing the lessons to them, so that inherently biases this analysis). That said, their pronunciation is muuuuuuuch better than ours.

I too often read about research that suggests children are better at learning languages than adults, which may or not be true, but I am rarely convinced by these reports. Finding a correlation between the age a person started learning a language and their fluency, the reports then conclude that children are superior learners, without considering other factors like that bi-lingual children are exposed to their languages constantly and must learn them to communicate with others, while adult learners find 2 hours of study onerous and suffer few if any consequences from not learning.

Other studies are more scientifically rigorous, testing the aptitude of various language learning abilities with quizzes and games, but I think we can infer very little about the efficiency of long term studying from these narrow tests.

Just anecdotally, I live in Japan, and most foreigners I know who come as adults and work in a Japanese environment are conversational in a year, fluent in 3 or 4.

Unfortunately I can't remember where I heard that recently. Knowing my own habits, it wasn't merely hearsay, and may have been from a podcast or other such non-fiction program, and still may have been a rogue opinion. I wish I could think of it. I'll Google around this evening.

For most people this is probably true, but I started learning Mandarin in college and now have a native-sounding accent, and I know others who have pulled it off too.

I think the key was that in the first few months of studying Mandarin, I listened to recordings of short phrases and repeated them back over and over again until I sounded exactly like the recording. I spent hours in my university's language lab with headphones on, doing this.

But you have to be able to imitate sounds that you hear, and I guess not everyone is naturally good at this. I'd imagine that people who are good at doing impressions, as well as perhaps people with musical talent, have an advantage at learning accents.

What I've gathered is that accents depend on your ability to distinguish phonemes - the classic example is that in western languages there is p vs. b sound - whereas in Tagalog there is also a pb intermediate sound.

If you have an ear for knowing that some languages use that phoneme, you will be more likely to be able to both hear and articulate these sounds (i.e., your aptitude for learning language accents is higher)