Does depression speed up brain aging? Or, is cognitive decline causing people to be depressed?

It’s the chicken and egg problem. Depression could accelerate cognitive aging. Or, our brain ages and itself is a risk factor for developing dementia. These two processes aren’t distinct, and there could also be a third factor so that could lead to both developing depression and impaired cognitive function and brain aging. Some examples include stress, which could pose the risk for both our mental, physical, and cognitive health.