The spirit does not age...it exists beyond the physical realm anyway. So the matter of "old and senile" doesn't make any difference.
Why must people be limited to the idea of life being bound and restricted by the human body? The human body dies. If someone believes in a soul/spirit that transcends death then why on earth attach physical attributes to something so unphysical?
The mind and the brain are two different things. While the brain may be affected and make the person appear to communicate in an abnormal way from most, does not mean that the essence of the mind is not healthy. The "mind", in essence, is the workings between the spirit and the body. The personality, the truth of a person.
The physical and the spiritual are NOT the same.