Chapter 9 in the book "Spark: The Revolutionary

New Science of Exercise and the Brain" is on aging. Here are two relevant paragraphs:

As we age, the cells throughout the body gradually lose their ability to adapt to stress. Scientists have yet to figure out exactly why this happens, but it’s clear that older cells have a lower threshold for combating the molecular stresses of free radicals, excessive energy demands, and overexcitability. And the genes responsible for producing proteins to clean up the damaging waste stop doing their job, which can lead to a cellular death spiral that neuroscientists call apoptosis. As the damage builds up, the immune system is activated and sends in white blood cells and other factors to mop up dead cells, which creates inflammation; if the swelling becomes chronic, it creates even more damaging proteins, and these are directly linked to Alzheimer’s.

In the brain, when neurons get worn down from cellular stress, synapses erode, which eventually severs the connections. With the decrease in activity, the dendrites physically shrink back and wither. Losing a signal here or there isn’t such a big deal at first, because the brain is designed to compensate by rerouting information around dead patches in the network and recruiting other areas to help with trafficking. There’s a certain redundancy built into the system. Remember, we’re talking about one hundred billion neurons, each of which might have up to one hundred thousand inputs. It’s a very social network that thrives on making new connections and, as I’ve mentioned, is constantly rewiring itself and adapting — provided there’s enough stimulation to spur the growth of new connections. As we get older, more real estate is required to carry out any given function. Wisdom, I think, is a reflection of how adept the brain is at compensating for this loss of efficiency.