As people with Down syndrome age, they face an increased chance of developing the brain disease called Alzheimer's (sometimes referred to dementia or senility).

Symptoms included such things as mental sluggishness, loss of concentration, poor memory, irritability, lack of confidence, excessive worry, hypochondria, depression, obsession, suicidal tendencies, and senility.

Of the remaining 398 deaths, 101 were attributed to senility; and for 297 deaths the cause was either ill-defined or unknown.

In general we find an analogy between the development of groups and of organs; we discover that each phyletic branch of certain organisms traverses a geologic career comparable to the life of an individual, that we may often distinguish, especially among invertebrates, a phase of youth, a phase of maturity, a phase of senility or degeneration foreshadowing the extinction of a type.