I have a little experience in this area. I would sum up the reasons as: 1. Children put in more time. An adult who puts in an hour a day of study is studying

I have a little experience in this area. I would sum up the reasons as: 1. Children put in more time. An adult who puts in an hour a day of study is studying hardcore, but a child will play with people or watch cartoons in a new language for hours and hours. When it comes to your native language you basically don't have a choice to use it for 12+ hours a day. 2. Most adults aren't used to learning new things. Children are learning new things all the time, it's their bread and butter. Adults often haven't learnt anything new since university age. Being a poor communicator in a new language is often very demotivating. 3. Adults can't shut off their native language. They're so used to speaking and thinking using their native language everyday that when it comes to a second language they can't shut it off. I find people who are good at suppressing their native language attain a far better level in the long run. Their are certain signs that someone is good at doing this, usually you can tell after a few meetings with them.