What about language acquisition? I think it's quite established that ability goes down with age. My experience seeing retired people (with plenty of free time) taking language classes is that they take quite a bit more time to achieve the same results. Pronunciation is especially hard to fix.
Adult language acquisition is a really weird problem.
tl;dr: Plenty of adults actually _can_ learn foreign languages if they put in the hours, though few have sufficient dedication. But maybe not everyone.
First, some background on my own experience: I started learning French a little about 2.5 years ago, past the age of 30. I can read non-fiction books in French, and about half the articles in Le Monde. Right now, I'm only studying French a few minutes a day, to maintain my knowledge while working on my startup. Later on, I'll go back to several hours of study a day. (I'm currently at ~350 hours of cumulative study.)
Here are some of things I've noticed about adult language learning:
1) You can be an enormously gifted language learner with no skill for accents. Alexander Arguelles, for example, has a extremely high degree of fluency in at least 6 languages, a solid command of another 12, and conversational abilities in many more. But his accents are generally terrible. In contrast, I have a decent ear for accents, and some people are amazingly gifted.
2) Learning a foreign language is a beastly amount of work: 250 hours for conversational ability (if you're gifted), 1000 or 2000 for moderate fluency, and several thousand more to get really good. Basically, if you're an adult and you're not fully immersed in the foreign language, it requires the kind of dedication you'd need to run a startup. Most people don't seem to have that kind of dedication.
3) It's possible to gain fluency in a foreign language past the age of 65. Google "Steve Kaufmann, polyglot", for example, who's still going strong. And there's a Japanese guy who picked up Chinese and Korean after 65, and who has started on French around the age of 100.
4) There is _some_ kind of declining neural plasticity, at least for some people. It's a truism that virtually every single 5-year-old gains fluency in the language spoken by their peers. But as people get older, a smaller portion of the population succeeds in learning a foreign language. And past 20, there will almost always be small defects that mark you as a (highly fluent) foreigner and not a native.
So if you're interested, and willing to pay a steep price in time and effort, you might as well go for it. I'd be happy to write up a post explaining how to make best use of your first 250 hours of study, if people are interested.
