| Under 50 years | Very rare, primarily genetic forms |
| 50-59 years | ~1% of AD cases |
| 60-64 years | ~5-10% of AD cases |
| Down syndrome | Trisomy 21 leads to APP overexpression; ~50% develop AD by age 60[^6] |
| Head trauma | History of traumatic brain injury increases risk |
| Cardiovascular risk factors | Diabetes, hypertension, hypercholesterolemia |
| Education | Lower educational attainment associated with earlier onset |
| Posterior cortical atrophy (PCA) | Visual processing deficits, often mistaken for ocular disease[^8] |
| Logopenic aphasia | Language-dominant presentation with word-finding difficulties |
| Behavioral variant FTD-like | Personality changes, disinhibition (can be PSEN1-related) |
| Corticobasal syndrome | Motor symptoms with asymmetric rigidity |
| Average delay | 2-3 years from symptom onset to diagnosis |
| Databases | OMIMOrphanetClinicalTrialsPubMed |