dementia becomes more common with age. about 3% of people between the ages of 65 and 74, 19% between 75 and 84, and nearly half of those over 85 years of age have dementia. the spectrum ranges from mild cognitive impairment to the neurodegenerative diseases of alzheimer's disease, cerebrovascular disease, parkinson's disease and lou gehrig's disease. furthermore, many types of memory decline with aging, but not semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady until late adulthood (see aging brain). intelligence declines with age, though the rate varies depending on the type and may in fact remain steady throughout most of the lifespan, dropping suddenly only as people near the end of their lives. individual variations in rate of cognitive decline may therefore be explained in terms of people having different lengths of life. there are changes to the brain: after 20 years of age there is a 10% reduction each decade in the total length of the brain's myelinated axons.