Over 50% of the Veterans enrolled for VA health care are over the age of 65. Dementia prevalence increases with age, and with the increase in the population of people ages 65 and older, the total number of people with dementia is also increasing. Older Veterans often have comorbid PTSD, major depression and traumatic brain injury so that they are at 2 to 5 times the risk for cognitive impairment and dementia compared to the general population. There is evidence that exercise interventions in sedentary older adults could improve both physical and cognitive function. However, there have been very few studies on the effects of exercise on cognition in older Veterans and do not reflect the broader ethnic and health-status diversity of Veterans. Thus, improved knowledge of the role of exercise on cognition as well as the predictive power of biomarkers could have a major beneficial impact on Veterans' functional independence and quality of life. The investigators hypothesize that participation in the VA Gerofit exercise program will improve cognitive function in older Veterans and that blood and muscle biomarkers will predict these improvements.