Jamieson, Fausset and Brown on Ecclesiastes 12:3

3. keepers of the house--namely, the hands and arms which protected the body, as guards do a palace (Ge 49:24; Job 4:19; 2Co 5:1), are now palsied.

strong men ... bow--(Jud 16:25, 30). Like supporting pillars, the feet and knees (So 5:15); the strongest members (Psa 147:10).

grinders--the molar teeth.

cease--are idle.

those that look out of the windows--the eyes; the powers of vision, looking out from beneath the eyelids, which open and shut like the casement of a window.