I saw all the living who move about under the sun, along with that youth who was to stand in the king’s place.
Solomon was himself a king, and therefore may be allowed to speak more freely than another concerning the vanity of kingly state and dignity, which he shows here to be an uncertain thing; he had before said so (Pro 27:24, The crown doth not endure to every generation), and his son found it so. Nothing is more slippery than the highest post of honour without wisdom and the people's love.
Commenting on Ecclesiastes 4:13-16
I considered all the living which walk under the sun,.... All men that were then alive, who were capable of walking upon the earth; even all of them that were under the heavens, in every land and nation, under whatsoever dominion or government: these, and their manners, Solomon had particularly observed, and made his remarks upon, by which it appeared how fickle the minds of...
"I considered all the living," the present generation, in relation to ("with") the "second youth" (the "legitimate successor" of the "old king," as opposed to the "poor youth," the one first spoken of, about to be raised from poverty to a throne), that is, Rehoboam. in his stead--the old king's.