Isaiah 40:30

“Even the youths shall faint and be weary, &c.] Such as are in the prime of their strength, and glory in it, yet through the hand of God upon them, by one disease or another, their strength is weakened in the way; or they meet with that which they are not equal to, and sink under, and are discouraged, and obliged to desist. Some think the Babylonians and Chaldeans are here meant, the enemies of Israel, and by whom they were carried captive. The Targum interprets this clause, as well as the following, of wicked and ungodly men; and so do Jarchi and Kimchi: it may be applied to the Heathen emperors, who persecuted the church of God, and were smitten by him, and found it too hard a work to…”

“young men--literally, "those selected"; men picked out on account of their youthful vigor for an enterprise.”

“Even the youths shall faint - The most vigorous young men, those in whom we expect manly strength, and who are best suited to endure hardy toil. They become weary by labor. Their powers are soon exhausted. The design here is, to contrast the most vigorous of the human race with God, and to show that while all their powers fail, the power of God is unexhausted and inexhaustible. And the young men - The word used here denotes properly “those who are chosen or selected” (בחוּרים bachûriym, Greek ἐκλεκτοὶ eklektoi), and may be applied to those who were selected or chosen for any hazardous enterprise, or dangerous achievement in war; those who would be selected for vigor or activity. The…”

“The youths - The youngest and strongest men, left to themselves. Isaiah 40:31”

“Isa 9:17, Isa 13:18; Psa 33:16, Psa 34:10, Psa 39:5; Ecc 9:11; Amo 2:14 Isaiah 40:31”

“Faith is all that is needed to ensure a participation in the strength (עצמה after the form חכמה), which He so richly bestows and so powerfully enhances. "And youths grow faint and weary, and young men suffer a fall. But they who wait for Jehovah gain fresh strength; lift up their wings like eagles; run, and are not weary; go forward, and do not faint." Even youths, even young men in the early bloom of their morning of life (bachūrı̄m, youths, from בּחר, related to בּכר, בּגר), succumb to the effects of the loss of sustenance or over-exertion (both futures are defective, the first letter being dropped), and any outward obstacle is sufficient to cause them to fall (נכשׁל with inf. abs. kal,…”