found in the relevant sections.
Prevalence
The prevalence of NCD varies widely by ag e and by etiological su btype. Overall preva-
lence estimates are generally only availabl e for older population s. Among individuals
older than 60 years, prevalence increases steep ly with age, so prev alence estimates are
more accurate for narrow age bands than for br oad categories such as “over 65” (where the
mean age can vary greatly with the life expect ancy of the given population). For those eti-
ological subtypes occurring across the lifespa n, prevalence estimates for NCD are likely to
be available, if at all, only  as the fraction of individuals who develop NCD among those
with the relevant condition (e.g., tr aumatic brain injury, HIV infection).
Overall prevalence estimates for dementia (which is largely congruent with major
NCD) are approximately 1%–2% at age 65 years and as high as 30% by age 85 years. The
prevalence of mild NCD is very sensitive to th e definition of the di sorder, particularly in
community settings, where evaluations are less de tailed. In addition, in contrast with clin-
ical settings, where cognitive concern must be high to seek and locate care, there may be a
less clear decline from baseline functioning. Es timates of the prevalence of mild cognitive
impairment (which is substantially congruent with mild NCD) among older individuals
are fairly variable, ranging from 2% to 10% at age 65 and 5% to 25% by age 85.
Development and Course 
The course of NCD varies acro ss etiological subtypes, and this variation can be useful in
differential diagnosis. Some subtypes (e.g., those related to traumatic brain injury or