If you waste your time enjoying the pleasures of life during your youth, wasting the power of your body and your sense organs, then if you want to reach your goal of merging with the Lord in your old age, you may not get that chance. There is no meaning whatsoever in serving delicious food to the demons, and then, when they have consumed everything worthwhile, offering the leftovers to God. Do you think that will please God? No! After all your powers and capacities have been dissipated by the demons of anger, greed, lust and pride, you try to offer God what little is left. But that offering will not be accepted by him. In this context, the Gita has emphasized that your youth is a very precious period which has to be used with great care to advance yourself spiritually.
When you have had something for a long time and taken it for granted, you may not appreciate its real value. It is only after losing it that you truly appreciate it. As long as you have your eyes, you do not know the value and the preciousness of your eyes. You only realize the importance of vision when you lose your eyesight. In the same way, when you have good health and all your faculties are in their full glory, you do not understand their true value. After having lost your health, and when your faculties have become impaired, then you repent and lament that all your capacities and powers are gone. But lamenting at that point is useless. During youth you have allowed the bad habits and bad traits to become your great friends and get deeply rooted within you. You squandered and misused the capacities that have been given to you, blindly following your sensual desires. Later, these bad habits and bad traits become your principal enemies in old age.
Most young people do not use their powers of discrimination properly. They do not try to sort out who is their true friend and who is their foe. If you follow only your senses and lower instincts, and have not developed your intelligence to understand the meaning of life, then is there any reason for calling you a human being? Should you not be called a mere animal? Once you understand the significance of human life and fill yourself with the noble qualities of a human being, your senses will no longer be able to confuse you.