I don't think you can make a blanket statement like this. People's ability to adopt new accents is surely as varied as their ability to learn anything new. On one end of the spectrum you have people who can't pronounce one foreign word correctly and on the other you have professional actors and impressionists who are complete linguistic chameleons. In his 40's, Hugh Laurie learned a perfect American accent to play House
, for example.
When I was in college I was studying French, German, Russian, and Spanish. Even at college age, 18-20, there was a huge spread of talent for the accent. It seemed impossible for some people to adopt new phonemes but others were incredibly good at it.