IMO one of the most prominent reasons - and one that I never see mentioned - that learning a language as an adult is more difficult is that the older you are, the more socially-unacceptable it becomes to point out grammatical and vocabulary errors. When you're a child, elders correcting you is so natural it scarcely even breaks the flow of conversation:
child: "Me and Tim went -"
adult: "Tim and I went."
child: "{pause} Tim and I went..."
As an adult, you simply do not get this kind of feedback. There's no chance in hell I would interrupt another adult to make the above correction. It's simply too disrespectful. And yet it is precisely this disrespectful interruption and correction that enables children to have tight feedback loops which result in fast, effective learning.
child: "Me and Tim went -"
adult: "Tim and I went."
child: "{pause} Tim and I went..."
As an adult, you simply do not get this kind of feedback. There's no chance in hell I would interrupt another adult to make the above correction. It's simply too disrespectful. And yet it is precisely this disrespectful interruption and correction that enables children to have tight feedback loops which result in fast, effective learning.