Most people believe that it's much easier to learn a language when you're a child. I claim this is a myth and I will explain why people think it.
The biggest factor is time spent per day. Pre-fluent children have basically nothing to do with any of their time except "play" with the fluent speakers aronud them, and as a result they are surrounded by fluent native speech for almost every minute of every day. Adults, on the other hand, often try to "learn a language" in a classroom for an hour a day. This has got to be a difference of 10 times just looking at the amount of time spent on average! And yet, do adults often take more than a decade of active study to reach fluency? Do they ever take more than two decades?
Similarly, most adults trying to learn languages are not spending most of their study time talking with natives. They spend a lot of it reading textbooks or web pages, listening to recordings or perhaps talking with other learners. None of these are as efficient a way to learn as talking with natives.
Another factor giving children an artificial advantage is their lack of fear. I hadn't actually realized this one until I read this article on the topic, but it's a good additional point. Children have little or no fear of embarrassing themselves with their lack of fluency, while this is frequently an obstacle for adult learners who are speaking with natives. This is a miniscule factor compared to the above, but it's still significant.
On the other hand, adults have the tremendous advantage not only of perhaps having more conscious motivation to learn, but of being able to organize and plan their learning intelligently. When they can't talk with natives, adults can use things like Quizlet, web search, digital dictionaries, and can learn grammatical concepts from explanation in their native language instead of purely through trial and error. They also can come at the task with a conscious understanding of another language, which is often extremely helpful. Many adult learners don't properly take advantage of these things, in part because they don't understand semantics very well, but when they do take advantage of all these factors, it's a huge boost. Children have to do almost all their learning by instinct.
I conclude that adults could actually learn languages much faster than children, if they learned them in a comparable environment. The idea that it's harder as an adult is purely a result of a fallacious comparison which does not isolate the factor it compares.
I am a living demonstration. I know a 3-year-old who's pretty close to fluent in English, and I've acquired a comparable level of fluency in Spanish after about 3 years of active study. The difference? Throughout those "active" years (I'm discounting the long periods of time where I completely dropped the study), I'd estimate that I've studied for an average of 20 minutes a day. And almost none of that time was talking with native speakers. The vast majority of it was reading and listening to recordings.
The reason I'm mentioning the empirical demonstration at the end is because it's against my principles to ask someone to take my word for what I just said. (I actually do have an obvious motive here, which is that if true this myth would seem to threaten my beliefs about the nature of consciousness.)