Researchers have identified some of the key changes in the aging brain that lead to the increased risk. The changes center on amyloid beta 42, a main ingredient of Alzheimer’s brain plaques. 
The protein, a natural byproduct of brain activity, normally is cleared from the brain before it can clump together into plaques. Scientists long have suspected it is a primary driver of the disease.
“We found that people in their 30s typically take about four hours to clear half the amyloid beta 42 from the brain,” said senior author. “In this new study, we show that at over 80 years old, it takes more than 10 hours.”
The slowdown in clearance results in rising levels of amyloid beta 42 in the brain. Higher levels of the protein increase the chances that it will clump together to form Alzheimer’s plaques.
The results appear online in the Annals of Neurology.
Scientists believe the brain disposes of amyloid beta in four ways: by moving it into the spine, pushing it across the blood-brain barrier, breaking it down or absorbing it with other proteins, or depositing it into plaques.
“Through additional studies like this, we’re hoping to identify which of the first three channels for amyloid beta disposal are slowing down as the brain ages,” author said. “That may help us in our efforts to develop new treatments.”
https://news.wustl.edu/news/Pages/Brains-ability-to-dispose-of-key-Alzheimers-protein-drops-dramatically-with-age.aspx