The New York Times > Magazine > In Search of Lost Time
Every brain has an intrinsic aging rate. Based on data on human lifespan and inferred aging rates (natural lifespan: 60-120 years, mean 90*) the range is +/- 33%, with aging starting around age 20. So by age 40 some lucky people have a brain similar to that of the average 33 yo, others resemble the average 47 yo. That's a significant enough span that we begin to notice winners and losers in the aging lottery. I have not read anything, by the way, that tells me that this aging rate can be significantly improved upon (though severe dietary restriction might help if started at age 20 -- note, however, than anorexics have fairly severe acceleration of brain aging, possibly due to the direct toxic effects of stress hormones).
Now add disease. Even a relatively mild concussion ages the brain significantly. Vascular disorders, infections, neurologic disorders, persistent stress, substance abuse, even social standing all come into play. Some primary dementing disorders, such as Parkinson's, begin to manifest. (It's unclear to me if Alzheimer's is a primary disorder, or a disorders of accelerated aging of the brain. The distinction may be subtle.)
The result is that by our 40s many of us worry about our cognitive capacities. This is particularly true of "knowledge workers", and probably less true of managers, CEOs, or politicians (political skills seem to be far more resilient than mere IQ).
This NYT Magazine article provides a fascinating summary of recent thinking about these cognitive disorders, and about the consequences of an aging brain in a post-industrial world.
It has an important message for those who blithely assume that extended life expectancy means we can work longer. They confuse life expectancy with functional cognitive skills. They'd never assume that brick layer would be laying bricks at 60, but they imagine the brain is less vulnerable to age than the spine. (Ok, so the spine is a pretty crummy device, and is a strong argument against "intelligent design", but you get the idea.)
So, yes, we'll be working when we're 75. We won't, however, be devising new implementations of cutting edge nanotech. We'll be doing the 2030 equivalent of bagging groceries -- except for politicians and the lucky few who will have the brains of a 55 yo at age 70.
Ten years ago it was obvious that the best way to dodge the demographic bullet would be to throw great resources into identifying specific interventions that might slow the aging of the human brain. We didn't do that. Too bad.
* Update: being middle-aged myself, I naturally flubbed the trivial arithmetic here.