As we age, our energy level slowly but steadily diminishes. Both body and mind. This increasingly becomes inconvenient, uncomfortable, and unhealthy.
Aside from attempts at temporary relief, the long-term solution is to identify and minimize those things in our daily life that drain energy unnecessarily.
These are different things for different people. One common factor is memory. The older we get the more things we have to remember. And the more space and energy is taken up in our mind by these memories. Sooner or later we have to pick and choose which memories we will keep alive and which we will allow to fade away, in order to free up space and energy for current activities. Senility is what happens to people who do not do this.
Inattentively keeping alive useless memories intensifies diminishment of energy. For instance, participating in, or viewing, trivia competitions. This is normally justified by the pleasure one takes in remembering anything. Contemporary culture regards memories as wealth; the more one remembers anything, the longer one remembers anything, the wealthier one is. Memories, since they take up so very much of the mind, are essentially considered to be the entirety of the mind. So that as aging necessarily reduces at least some of the memories, we generally consider ourselves to be losing our mind.
The intense reluctance to discard any memories comes from vigilant maintenance of our strongest and longest memory: who we are. Our sense-of-self, center, core, ego. Those with the smallest sense-of-self are the least likely to keep alive useless memories and thus become senile.