Your clothes are soaked with sweat. You are bent over like an old man. I am fresh as a cucumber. If I changed places with you, I would have to start all over again filling that empty sack of yours. This is the Torah's perspective.
Life has a destination and goals, things to be accomplished, growth to be achieved.
Therefore, age rather than youth must be venerated. Each year of life yields wisdom and experience that the most accomplished young person cannot possibly attain. It is true that youth is bursting with strength and vigor, but a person's worth is not to be measured by physical endowments. The body is but an accessory of the soul, and the spiritual growth of old age enriches the soul. Our matriarch Sarah lived with this perspective.