As is my wont, I'm listening to a book on CD in my car. I usually favor history and lectures and such, because the continuity isn't as important as with, say, fiction. Today I was running errands, listening to a lecture on language. The lecturer is going over some of the evidence for/against the idea that grammar is innate/inborn in humans, and as part of his discussion, he's talking about how very easy it is for little kids to learn language compared to adults. Yes, he basically says that little kids don't even have to work at learning language.
But here's the thing. If you've spent time with little kids, you realize that they're working really hard for hours every day learning language, learning to manipulate the world, and so on. I think we're disrespecting little kids somewhat if we don't recognize that they're working really hard at all the learning they're doing. It looks like it's not work, and hopefully there's plenty of fun and play involved, but even that is brain work.
If I spent 8+ hours a day constantly having people talking to me within a context, giving me feedback on my attempts to talk, I'd learn a language more quickly than otherwise. In fact, I learned Spanish in an immersion program as a young adult, and within a couple months could hold a pretty decent conversation. I didn't sound great or perfect, but I could hold a basic conversation with someone who was willing to be patient, and many people were willing. It was exhausting, too, way more than you might think.
One of my colleagues went to Nicaragua last year to work on her Spanish, doing an intensive immersion program, living with a family, and so on. She told me about one day when she just broke down and started crying (worrying her host "mother" greatly in the process). She told me it made her think about when toddler's get so upset about something that seems unimportant because she was just learning a lot of stuff, working hard, being very tired, and very frustrated by not being able to express herself. And like a toddler (according to her), she just got really upset and started crying over something that wasn't that important.
All this has nothing to do with whether it's harder to learn a language as an adult. It may well be. But we rarely learn languages as adults the way we did as children. (And, for most of us, as an adult, we're learning a second language, so we know to think about things like grammar and such. It may be much harder to learn grammar and such for a first time if you've never learned language, I suppose.)
The lecturer also talked about accents, but I think that's a bit of misdirection, since we all have accents within our native language, and learning a specific accent in a second or third language may be harder because we've learned to make certain sounds very young. (Or so a colleague tells me: an infant begins to sort out and imitate specific sounds from that language/s they're exposed to really young.)