Amyloid-beta accumulation in the brain occurs approximately 20 years before the onset of clinical symptoms of Alzheimer's disease.

Study: Predicting accumulation and age at onset of amyloid-β from genetic risk and resilience for Alzheimer's disease

This study shows that the sticky amyloid protein builds up in the brain many years before people start forgetting things or showing signs of Alzheimer's — which matches the idea that it starts about 20 years earlier.

Study: Sleep duration and amyloid status moderate the association between mood symptoms and amygdalar tau in preclinical Alzheimer's disease

This study found that some older adults have amyloid plaques in their brains but still think and remember normally — meaning the plaques build up long before memory problems show up, which matches the claim.

Study: Neuropsychiatric Profiles and Cerebral Amyloid Burden in Adults without Dementia

This study found that people who haven’t been diagnosed with Alzheimer’s yet but are worried about their memory or whose caregivers notice they’re anxious or sad often have high levels of amyloid-beta in their brains—meaning the disease process starts many years before symptoms show up.

Study: Investigating the Aβ and tau pathology in autosomal dominant Alzheimer’s disease: insights from hybrid PET/MRI and network mapping

This study found that people who will later get Alzheimer’s start building up a harmful protein called amyloid-beta in their brains about 20 years before they show any memory problems — like a silent warning sign that starts long before symptoms appear.