Your brain has its own immune system. It runs on specialized cells called microglia whose job is to protect neural tissue, clear cellular debris, and respond to threats. Under normal conditions they do this efficiently and then stand down.
After 50, something changes.
Chronic low-grade inflammation, the kind that builds quietly from decades of ordinary life, begins reaching the brain through the bloodstream. When it does, microglia activate. And when they stay activated over months and years, they stop being protectors and start causing damage. They release inflammatory signals into brain tissue. They interfere with neural connections. They accelerate the accumulation of the proteins most closely associated with cognitive decline.
This process is called neuroinflammation. It has no early pain signal. No obvious symptom that sends you to a doctor. It just runs.
At the center of this process is a molecular switch called NF-kB. When NF-kB activates chronically, it drives inflammation throughout the body and brain. In your joints it shows up as stiffness.
In your brain it shows up as the gradual mental slowness most people accept as an inevitable feature of getting older.
It is not inevitable. It is a mechanism. And mechanisms can be addressed.