Aging is a chain of additive processes, starting with damage at the level of molecules and cells caused by your metabolic processes, damage that accumulates like biochemical rust. Eventually there's enough rust to harm the working of critical systems at larger scales, such as small blood vessels, or the liver, or the muscles of the heart. Malfunctions and lost efficiency in those systems cause and accelerate problems in other organs, like the brain:
a third of the risk for dementia (33 percent) was associated with damage to the brain from small vessel disease. .. this small vessel damage is the cumulative effect of multiple small strokes caused by hypertension and diabetes, strokes so small that the person experiences no sensation or problems until the cumulative effect reaches a tipping point.
The rust that causes blood vessels to degrade in performance probably has to do with increasing oxidative damage, the accumulation of AGE compounds that interfere in cell signaling, and possibly the chemistry of chronic inflammation. Blood vessels are complex little machines that rely on their structure and a coordinated set of signaling mechanisms to do their job. As the damage of aging changes that structure, interferes with signaling, and degrades the effectiveness of our blood vessels, this in turn hurts the rest of our biochemical systems.