I'm not saying I'm the best informed on this topic, but I thought the root cause has been known for a long time now as degraded endocrine and cardiovascular function.
That's also why Alzheimer's can take so long to develop. It's just one aspect that we've chosen to focus on because it's more clearly noticeable, but it cannot easily be treated in isolation from everything else. If it was, it would regress quickly without fixing the root causes.
There is no single root cause. Many scientists have preferred to ignore this fact and that has been a serious problem. Everyone likes a simple story. Age-related diseases are not simple stories.
We truly do not know the root cause. There are plenty of folks with "degraded" endocrine, cardiovascular, and both systems. Most of them do not develop Alzheimer's.