I live in Japan and speak Japanese reasonably well (not native level by any stretch of the imagination, but I can have casual conversations in virtually any normal context). I know a lot of expats who live near me and just can't seem to learn Japanese. One of the biggest things I noticed they had in common was a desire to avoid being treated like a child. My level is like that of an 8 or 10 year old (though I lack fluency is some normal grammar which most children have no trouble with). I'm very comfortable acting like an 8 or 10 year old. When I go to the doctor for example, I don't expect to understand what he's saying. The "explain it to me like I'm five" thing is my ordinary lifestyle. But I find that some people just can't accept the step down. If they are 30 years old, they want to go out with native speakers and interact as a 30 year old. Then they get frustrated and humiliated when they can't understand what's going on. They want to watch the news on TV and get frustrated when they can't understand any of it. They refuse to watch things (like cartoons) that they can understand, because it is too childish. It's a bit of a blocker, I think.

Admittedly, though, it's frustrating that people always have to make an effort with you. If you go out with a group of native speakers to a bar for an evening, there's a good chance that the only conversation you understand is when someone makes a special effort to speak to you. There is nobody in your peer group that has the same communication level as you, so you can get really lonely. I always had the strategy of coming up with one-liner jokes. Just sit back and listen and listen and listen until you finally understand something and then think of a quick funny, but irrelevant thing to insert in the conversation. It's doubly funny because everybody thinks you have no idea what's going on. But that was my thing for years -- I never said anything meaningful in a conversation in a group. Just like a kid...

But all that aside, it's actually pretty easy to show that adults can learn languages as fast as children. Children acquire about 3 "word families" per day on average from the age of 1.5 up to 15 (and continue as long as they continue formal education). A "word family" is a bit like a piece of vocabulary, but includes all the inflections and combination words (so if you know "police" and "station", then you also know "police station" -- it's not counted as a new piece of vocabulary). This works out to about 1000 word families a year (although the rate is very variable between the ages of 1.5 and 5 -- the average child ending up with 5000 word families by the age of 5).

That kind of stuff is very well understood. You can learn faster than that as an adult. It's not even very hard. The trick is understanding that it's going to take up to 5 years to speak like a 5 year old. Most people forget what it's like to be 5 and how crappy your language skills are. They think, "Oh these 5 year olds can play easily together and jabber on constantly" -- but put them in an adult environment and the vast majority will clam up or be just as frustrated as we are.