Tiruin, It's all about the size of the bucket. A person can learn a lot of stuff later in life, but they literally don't have the absorptive potential of the young.
A 50 year old can learn a bunch of new stuff when they're old, but it usually just replaces 'dead information' which they gathered when they were young. Like pouring new water in a bucket full of water.
Or so is how my professor explained it to me.
Mm... iirc, it's not so much information -- we've got memory storage capacity that would last for something like several hundred years or something equally ridiculous -- as mental pathways. Connections
between ideas (or parts of the brain, or some such), or something along those lines. Which becomes harder and harder to form as people grow older, especially past the whole physical maturity peak thing, before which they form much,
much easier.
As you note, though, they
can be re-purposed and whatnot -- it's one of the reasons it's usually easier to teach older folks if you can relate whatever you're teaching them to things they already know, or modes of thinking they're already used to. Mind you, that helps the young as well, but from my understanding of things younglings're better at going
without that.
And @ T's family -- again, that doesn't mean something silly like the elderly stop being able to learn or whatev'. You can still have very mentally spry old people, and folks that learn very well once the brain's mostly done growing. Just means they're not
as equipped for it as they
were, from a neurological perspective, and on the average you're going to see older folks have a harder time learning new things. Barring neurological damage they can still do it, just not quite as easily.