Phase 1: Young healthy men and women between 18 - 30 years of age, and older men and women between 50 - 80 years of age. Phase 2: Men and women living in Tarrant County, Texas, between 50 - 80 years of age. In addition, the investigators will study up to 120 younger men and women, between 18-30 years of age, equally divided by race/ethnicity (same as above) and sex, to serve as reference controls.

Phase 1: Clinical diagnosis of dementia; major depression (diagnosed by a physician), visual or auditory impairments that preclude neuropsychological testing or otherwise jeopardize the health and wellbeing of the participant during testing, non-ambulatory status (i.e. less than 10 m without mobility aid), shortness of breath while performing normal activities of daily living (walking through grocery store), body mass index > 45 kg/m2, heart failure or cardiomyopathy, acute coronary syndrome, primary valvular heart disease, severe gait disorder due to musculoskeletal disease or neurological motor deficit (e.g. severe osteoarthritis, lower limb amputation, Parkinson's disease, major stroke with motor sequelae), the use of neuroleptics, participation in a dedicated weight loss program within 6 months before the study; daily consumption of more than two alcoholic beverages; current treatment for cancer, and estimated life expectancy <4 years. Inclusion/exclusion will be verified using the demographics and medical history. Phase 2: Clinical diagnosis of dementia; major depression (diagnosed by a physician), visual or auditory impairments that preclude neuropsychological testing or otherwise jeopardize the health and wellbeing of the participant during testing, non-ambulatory status (i.e. less than 10 m without mobility aid), shortness of breath while performing normal activities of daily living (walking through grocery store), body mass index > 45 kg/m2, heart failure or cardiomyopathy, acute coronary syndrome, primary valvular heart disease, severe gait disorder due to musculoskeletal disease or neurological motor deficit (e.g. severe osteoarthritis, lower limb amputation, Parkinson's disease, major stroke with motor sequelae), the use of neuroleptics, participation in a dedicated weight loss program within 6 months before the study; daily consumption of more than two alcoholic beverages; current treatment for cancer, and estimated life expectancy <4 years. Inclusion/exclusion will be verified using the demographics and medical history.