<p>Okay, so I think that googles into "brain aging + fluid intelligence" would be helpful. I'll do that now.</p>
<p>There are several stages in brain aging, so to speak. The first stages actually start as the brain is developing. There is a critical period for language development - and if people learn a language after that particular age/period, they will never become non-accented speakers in such a language. Of course, this says nothing about openness or fluid intelligence. </p>
<p>The brain pretty much stops growing in the teenage years. It starts to shrink after age 20. That alone says nothing, of course. Just saying that the "brain starts to degrade" is faulty logic - one has to look into specifics.</p>
<p>The other concern is that the protective measures against Alzheimers only apply to how much you've developed your brain up to age 30. After that age, brain exercises exert no more neuroprotective factor. I have to look up the link citing that again though. Found it: <a href="http://www.slate.com/id/2165040/pagenum/2/%5B/url%5D">http://www.slate.com/id/2165040/pagenum/2/</a> (slate isn't the best source, but whenever it points out skepticism towards some actions that people are taking - that skepticism is often well-taken)</p>
<p>Crystalline intelligence should logically decline as already-formed synapses start to degrade, whereas fluid intelligence should logically decline as synapses start to form slower and slower (that should be the type of research I should take my google into next). </p>
<p>Let's see...
<a href="http://intelligencetesting.blogspot.com/2006/12/age-decline-in-reasoning-speed-and.html%5B/url%5D">http://intelligencetesting.blogspot.com/2006/12/age-decline-in-reasoning-speed-and.html</a></p>
<p>==
The other concern is more of a logical one. Scientists tend to be very inquisitive types through life. Do you think that their curiosities (or passions for intensive mental training) decline after a particular age? It's hard to say. It's true that motivation often declines after a particular age. One scientist invoked the hypothesis that males are particularly motivated to grab accomplishments early on due to the prospect of attracting mates (I'm dubious of the hypothesis, but it's an alternative possible explanation, which cannot be ruled out yet).</p>
<p>==</p>
<p>Okay, so...</p>
<p>
[quote]
“Fluid intelligence (also called "native mental ability") is the
information processing system. It refers to the ability to think and
reason. It includes the speed with which information can be analyzed,
and also includes attention and memory capacity."
[/quote]
<a href="http://answers.google.com/answers/threadview?id=203714%5B/url%5D">http://answers.google.com/answers/threadview?id=203714</a></p>
<p>==
Again, I realize that the sources are not particularly reliable. I'll have to look more into it. They cite reliable sources and there seems to be no particular incentive for them to cite biased sources (as in, they don't seem to be advertising any anti-brain aging products).</p>
<p>Some personality styles are more likely to decline with fluid intelligence than other personality styles. Yet, the personality style of the scientist/mathematician (open, inquisitive, willingness to learn anything that would help with solving problems) is the one that is the least likely to decline with fluid intelligence. However, even scientists differ in that trait, some of them being better than others.</p>
<p>==</p>
<p>Now, one's personality and learning style most certainly cannot change one's capacity to learn/remember/be attentive in a particular task/etc. This is partially counterbalanced by a person's ability to learn (what one psychologist calls "learnable intelligence) what strategies are best used with one's learning style (better strategies allow one to learn more and to apply that knowledge with more accuracy). However, the person should eventually learn which strategies are best used with one's learning style (and cannot learn any more beyond a certain point). After that, there is only one direction, and that direction is in that of decline.</p>
<p>Fluid intelligence deals with measures on IQ tests that are culturally independent. There are some interesting correlations (though not strong ones - reaction time has a 0.5 correlation with IQ, we still don't appreciate the full significance of that correlation). </p>
<p>So what is the issue then? The making of novel discoveries. There are several ways to make novel discoveries. (1) is in the synthesis of vast amounts of knowledge (dependent on crystallized intelligence). (2) is the "ingenious" step of making the right logical leap in a theoretical physics/math problem. The question is, does (2) decline with age? One has to look at the empirical evidence of that. Dean Simonton (1988) has found some empirical evidence for that trait in his book "Scientific Genius." (which is consistent with the paranoia of mathematicians/physicists). Novel discoveries in the social sciences, on the other hand, which are dependent on synthesizing vast amounts of knowledge, depend more on crystallized intelligence. As for the neurological mechanisms, while the brain does decline after age 30, we still do not appreciate the full significance of its decline as of yet. As you pointed out, that statement alone can apply to crystallized intelligence. It can only be applied to fluid intelligence as distinct from crystallized intelligence once we figure out which neuromechanisms decline faster than others after age 30.</p>