A gene related to Alzheimer's disease may start to show effects on brain structure and mental sharpness as early as preschool, a new study suggests.

Researchers have long known that a gene called APOE is related to the risk of developing Alzheimer's disease. People who carry a variant of the gene known as e4 have a higher-than-average risk.

The new study confirms what smaller studies have hinted: The gene's effects may be apparent even in early childhood.