The prevalence of dementia at given ages has decreased compared to decades ago. Today's 90-year-olds have less than half the dementia risk compared to those in 1984.
Research indicates that the heritability of human lifespan is approximately 50% when using a redefined concept of heritability. This suggests genetic factors account for about half of lifespan variation, while environmental and other factors contribute the remaining half.
Heritability measures how much of the variation in a trait within a population is due to genetic differences. It does not indicate how much of an individual's trait is determined by genes versus environment. The concept is often misunderstood in discussions about intelligence, personality, and other complex traits.