A little speculation about neuroplasticity. Decline in language learning skills, starting with adolescence is like a rule of thumb, stated to display decline in neuroplasticity - neurons get less and less capable of creating new connections.
They say brain deteriorates with age, loses weight each year. However, I know very energetic 75 - 80 year-old persons with sharp minds, good reflexes (good car drivers) and having excellent memory.
While language learning difference in fluency for the very early "critical" years is obvious, I suspect for the adulthood there's a factor which is underestimated, I'd call it "neuro-competition", it's supposed to be there in the early age either - initially there are not that much competing patterns.
With time brain has too many existing patterns and there are too many activities and goals which brain have to think about (such as you're unable to concentrate on this for long enough), so it's harder to invest enough resources and to justify that new patterns worth replacing the old ones.
If you learn intentionally and systematically many languages, polyglotism may happen to be not that hard that it seems (at least for basic communication level). Languages are grouped in families, common rules are noticed, there is "transfer of learning" between them and after some time new languages are supposed to come easier.
In much simpler form this happens with programming languages - beginners think it's a big deal to know many languages, but the more you learn, the easier you learn new ones.
A funny polyglot :)http://www.youtube.com/watch?v=8LJi47Kzvv8&feature=fvw