Scientists know Alzheimer’s disease gains a foothold in a patient’s brain years before he or she first stashes car keys in the freezer, or gets lost coming home from the store. But a new study suggests the changes in the brain that set the stage for Alzheimer’s may start decades — indeed a lifetime — before symptoms appear.

The latest research found people as young as 20 have detectable levels of beta-amyloid molecules — the building blocks of the amyloid plaques that are a key physical sign of Alzheimer’s — in a group of brain cells that come under attack in Alzheimer’s disease.

In older people who were cognitively normal at the time of their death, researchers also found amyloid molecules, usually in greater concentrations and often already clumped together, in those special neurons.

The scientists looked specifically at the cholinergic neurons in the basal forebrain, which are among the first to sustain damage and to die off in Alzheimer’s disease. Researchers found that all of the brains they studied tended to accumulate more amyloid molecules — and more “clumps” of the sticky protein — inside those cells with age.

Their findings appear in the journal Brain.

“What this suggests is that Alzheimer’s disease is truly a lifelong process,” said the study’s lead author, Changiz Geula, a neuroscientist at Northwestern University’s Feinberg School of Medicine.

“If we were to try to prevent the formation of clumps in this population, these findings would suggest we would have to intervene when a person is much, much younger,” he added.

At very low levels of concentration, amyloid molecules appear to be normal and perhaps even perform some valuable function in the basal forebrain’s cholinergic neurons, Geula said in an interview. Over time, their concentration appears to build, and they form clumps called oligomers in those cells.