Senile, a.
Etymology: [L. senilis, from senex, gen. senis, old, an old man: cf. F. sénile. See Senior.]
Of or pertaining to old age; proceeding from, or characteristic of, old age; affected with the infirmities of old age; as, senile weakness. "Senile maturity of judgment." Boyle. Senile gangrene (Med.), a form of gangrene occuring particularly in old people, and caused usually by insufficient blood supply due to degeneration of the walls of the smaller arteries.
Shades of meaning
Fernald has no entry of its own for senile; he lists it under old.
Where he writes about it:
Decrepit, gray, and hoary refer to the effects of age on the body exclusively; senile upon the mind also; as, a decrepit frame, senile garrulousness. — from old
One may be aged and neither decrepit nor senile. — from old