[kyllo]

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.

[sampo]

Some people can also pick up accents without knowing the languages: http://www.youtube.com/watch?v=ybcvlxivscw