Why do people think that only kids can learn new accents?
Most people incorrectly think that only children are capable of learning new sounds. It's true that children are able to pick up new sounds more naturally, but adults have similar capabilities. The problem is regular English teachers have no idea how to help adults hear subtle sound differences. Only English teachers specifically trained in pronunciation know how to give listening training.
In an experiment conducted on Japanese ESL learners, researchers found that adults were able to hear sound differences they couldn't hear before when they were given proper listening training exercises. The adults could hear 80% better than before as long as they were given listening training with feedback. Isn't that amazing? I've found with my students that after a little bit of listening training they are able to hear differences they couldn't hear before in minutes. They are always amazed at their new superpower!