Adam Clarke on Job 12:20

He removeth away the speech of the trusty - The faithful counsellor and the eloquent orator avail nothing: Quos Deus vult perdere, prius dementat; "God infatuates those whom he is determined to destroy." The writer might have had his eyes on Isa 3:1-3, which the reader will do well to consult.

The understanding of the aged - זקנים zekenim signifies the same here as our word elders or elder-men; which includes in itself the two ideas of seniority, or considerably advanced age, and official authority. These can do no more to save a state which God designs to destroy, notwithstanding their great political wisdom and knowledge, than the child who can neither reason nor speak.