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Challenges make our brains smarter as we age

Ten years ago, right after I delivered a speech on innovation to the top tier of executives at a Fortune 100 company, one of the executives sought me out with a compliment. "Gee, Eric you haven't lost a step at all."
The executive, in his forties, had known me for a couple of decades-and what he really meant was that, in my late fifties, I hadn't slowed down mentally nearly as much as he expected I would.
Now in my late sixties, every other week or so, I experience this sort of attitude from people of all ages with comments like "Wow, you're still doing that?!" or, more subtly and insidiously, having my opinion discounted because of my age. So, yeah, I'm not happy about age bias-but is it at all grounded in reality? Do we lose a step, or two, or three as the decades accumulate?
Let's address that question by starting with some of the latest neuroscience on brain aging, and there is some good news.
Whereas brain scientists used to think we lost about one per cent of our brain cells every year after our late 20s, it turns out these estimates were based on methodological errors. Current estimates are that healthy brains lose only about 4 per cent of neurons by the time they get to their 70s and 80s.
The conventional wisdom that we don't replace dead neurons with new ones as we age has also proven incorrect. Finally, the brain retains its plasticity well into old age - growing, like an exercised muscle, with mental and physical exercise. For instance, older subjects taught to juggle showed increases in the volume of motor cerebral cortex (coincidentally, this physical exercise also increased cognitive function).
So that's the good news.
The other side of the story is that, on average the speed of cognition, including reaction time, does slow down in healthy brains as they age, and as we get older we generally have a harder time "inhibiting" (tuning out distractions while we concentrate). So on average, we don't get "dumber" as we age-but numerous replicated studies reveal we do take longer to be as smart as we always were and we have a harder time concentrating.
Although the research demonstrating cognitive slowing and age-related deficits is quite solid, I question the true cause of such declines, and whether they are completely inevitable.
Many studies suggest that staying both mentally and physically active can stave off cognitive decline, but I suspect there may be more subtle reason why some people experience cognitive decline as they age.
And that reason is expectation bias, also known as the Pygmalion effect or Rosenthal effect. Back in the 1960s,
Dr Robert Rosenthal of UC Riverside showed that when teachers were told that random groups of students had high or low IQs respectively, students who were given fictitiously high IQ at the beginning of the school year actually scored higher on real IQ tests at the end of the year than students with fictitiously low IQs. In other words, teacher expectations created a self-fulfilling reality: we can be as smart or dumb as people think we are.
Expectation has other powerful effects, such as the placebo effect and the nocebo effect. So, taking research on expectation into account, and having aged myself, and witnessed the effects of aging on those around me, here's what I suspect contributes to some of the cognitive decline shown in tests of elderly subjects: self-fulfilling prophecy.
It's hard to imagine that, given the power of the Rosenthal/Pygmalion, placebo and nocebo effects, that all of the stereotypes we've heard our whole lives about how people inevitably slow mentally with age haven't created the self-fulfilling prophecy of cognitive decline as we age. These effects can arise both from external expectations of others and with internal expectations.
The sad reality is that both types of expectations, those of others and of ourselves, can negatively affect our physical brains, producing the very results we expect. If others don't give us mentally challenging problems to solve, or we don't mentally and physically challenge ourselves because we think we're over the hill, then abundant research suggests that our brain volumes will actually shrink, fostering real cognitive slowing.
-Psychology Today 
Eric Haseltine is a neuroscientist and the author of Long Fuse, Big Bang