Ecclesiastes - Chapter 12
Osteoporosis
Solomon advises the reader to remember G-d while we are young and still have the ability to pursue His ways. If we wait until we are old and starting to become infirm, a person will not have the wherewithal to do so. We should do so before the sun, the light, the moon and the stars all dim and the clouds roll in. (The dimming bodies of light correspond to various parts of the head and face, as well as the soul, in one's declining years.) Do this before the caretakers of the house tremble, the strong men are afflicted with cramps, those who grind stop because of attrition, and those who look out the windows go dark. (The caretakers are the ribs, which protect one's organs. The strong men are the legs, the teeth grind and the eyes see.) The doors will be closed when the sounds from the mill are low, a person arises when he hears a bird sing and the sounds of instruments are hushed. (The doors are the bodily orifices and the mill is the digestive system. When a person becomes older, he sleeps more lightly, so even a bird can awaken him and his hearing starts to go.) When a person gets older, he's afraid to go out because he could fall and easily break a hip (see Rashi on verse 5). Old age springs upon a person suddenly, like an almond tree blossoming. His bottom drags behind him (compared to a grasshopper limping along - see Rashi on verse 5 for this one, too) and a person loses the sexual urge. A person then goes to his final destination and others mourn his departure.
And so, the silver cord breaks (i.e., the spine), as does the golden fountain (that is, the genitalia), the pitcher shatters (the stomach) and the wheel crumbles (that is, the eye). A person's body decomposes and his soul returns to G-d.
According to the Metzudas Dovid, the following conclusion was inserted by the court of King Hezekiah, who edited the Book:
And so, says Koheles (i.e., Solomon, who gathered all this wisdom), all of man's pursuits are pointless. Since he was wise, Solomon taught the people. He learned much, he instituted safeguards for the Torah and he composed the Book of Proverbs. He delved into Torah, which he faithfully transmitted. Be advised to keep the Torah, both Written and Oral - there's so much that it could never all be written down and one could spend a lifetime trying to master it.
The bottom line is this: defer to G-d and follow His word because that is a person's entire reason for being. G-d will repay a person for every action, even those done in secret, whether they be good or bad. (As with Eicha, the Book of Lamentations, you may find the penultimate verse repeated, to avoid ending on a negative note.)