Surah 36. Yasin, Ayah 68

I.e., man should never postpone his exercise of moral choice - for if human beings are superior creatures inasmuch as they have been endowed with the faculty of discernment and a wide measure of free will, let them also remember that "man has been created weak" ( 4:28 ) and liable to decline still further in old age, so that the time at his disposal is short.

 Human beings are born weak, then they reach their prime, then they grow weak after a certain age. See 30:54.

This connects on with the last verse. Everything is possible with Allah. If you doubt how man can be transformed from his present nature, contemplate the transformations he already undergoes in his present nature at different ages. As a child powers of mind and body are still undeveloped. As he grows, they grow, and certain moral qualities, such as courage, daring, the will to conquer, unfold themselves. In extreme old age these are again obscured, and a second childhood supervenes. The back of the man who walked proudly straight and erect is now bent. If these transformations take place even in his present nature and constitution, how much easier was it for Allah to cast him in an immobile mould? But Allah granted him instead the high possibilities and responsibilities referred to in the last note.