Jamieson, Fausset and Brown on Ecclesiastes 11:9

9. Rejoice--not advice, but warning. So 1Ki 22:15, is irony; if thou dost rejoice (carnally, Ec 2:2; 7:2, not moderately, as in Ec 5:18), &c., then "know that ... God will bring thee into judgment" (Ec 3:17; 12:14).

youth ... youth--distinct Hebrew words, adolescence or boyhood (before Ec 11:10), and full-grown youth. It marks the gradual progress in self-indulgence, to which the young especially are prone; they see the roses, but do not discover the thorns, until pierced by them. Religion will cost self-denial, but the want of it infinitely more (Lu 14:28).