Why is it said that "youthful ambition is irreplaceable"?

Bai Juyi
Age 16: Wild fires burn but never die; spring winds blow and they grow again
Age 45: As evening comes and snow threatens, would you join me for a cup of wine?

Li Bai
Age 43: Ten steps to kill a man, a thousand miles leaving no trace
Age 61: This shames even the woman who helped me; three times I've declined her kindness and couldn't eat

Li He
Age 17: I answer your golden terrace's call, wielding my jade dragon to die for you
Age 24: I don't know how high the sky is or how thick the earth, seeing only how moon's cold and sun's warmth torment life's years

Lu You
Age 46: On horseback I strike the wild barbarians; dismounted, I draft military dispatches
Age 85: Dead, I've always known all things end, but I grieve not seeing nine provinces united

Liu Yuxi
Age 33: A crane soars above clear skies through clouds, drawing my poetic spirit to azure heights
Age 54: Past the capsized boat, a thousand sails pass; before the sick tree, ten thousand kinds of wood spring forth

That youthful spiritedness and sharpness,
That exuberant vitality of "I can support the sky even if it falls,"
Once repeatedly ground down by time and reality,
truly becomes difficult to grow back.

Youthful ambition is truly something irreplaceable.