There is a reason why Solomon tells young people to remember God when they are still young (Ecc12:1); it's because youth is the time when one has great opportunities to love God truly with all one's strength. It's the time when one can choose to use his strength for the Master. To know God at an old age is fine; however, then strength is weak, the keepers tremble, and the grasshopper is a burden (Eccl.12:3-5). But, in the robust vigor of youth, one can truly speak of loving God with all one's strength. In his youth, David could talk of God as the one who trained his hands for war so that his arms could bend a bow of bronze and his feet could run against a troop and leap over a wall (Psa.144:1; 18:29,34). But, in his old age, he could not speak of being able in the same manner. David had the blessing of serving the Lord with all his strength in his youth.
There is no sadder sight than a young person giving up all hope saying, "I can't, I have no strength!" Some find repose in the teaching of a weak human nature. But, the Bible commands the young to be strong (2Tim.2:1), and positively says: "I have written to you, young men, because YOU ARE STRONG, and the word of God abides in you, and you have overcome the wicked one" (1Jn.2:14).