As someone who learned several languages both as a child and an adult, I've always thought the "kids learn new languages so easily" meme was rubbish. The children this refers to are typically thrown into an immersion environment where they spend 6+ hours a day with the target language with no way out of it (other than sitting around daydreaming, I guess). I've been in that environment several times.
Adults are very good at avoiding those kinds of environments. They'll study casually for an hour a day, carefully avoid situations in which they would be forced to use the target language, and then marvel at how much faster their kids are picking up the language. No surprise there!