Decrepit, a.
Etymology: [L. decrepitus, perhaps orig., noised out, noiseless, applied to old people, who creep about quietly; de- + crepare to make a noise, rattle: cf. F. décrépit. See Crepitate.]
Broken down with age; wasted and enfeebled by the infirmities of old age; feeble; worn out. "Beggary or decrepit age." Milton. Already decrepit with premature old age. Motley.
Note: Sometimes incorrectly written decrepid.
Shades of meaning
Fernald has no entry of its own for decrepit; 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