Allah tells us that the longer the son of Adam lives, the more he becomes weak after being strong, and incapable after being able and active.
Allah tells:
وَمَنْ نُعَمِّرْهُ نُنَكِّسْهُ فِي الْخَلْقِ ...
And he whom We grant long life -- We reverse him in creation.
This is like the Ayah:
اللَّهُ الَّذِى خَلَقَكُمْ مِّن ضَعْفٍ ثُمَّ جَعَلَ مِن بَعْدِ ضَعْفٍ قُوَّةٍ ثُمَّ جَعَلَ مِن بَعْدِ قُوَّةٍ ضَعْفاً وَشَيْبَةً يَخْلُقُ مَا يَشَآءُ وَهُوَ الْعَلِيمُ الْقَدِيرُ
Allah is He Who created you in (a state of) weakness, then gave you strength after weakness, then after strength gave weakness and gray hair. He creates what He wills. And it is He Who is the All-Knowing, the All-Powerful. (30:54)
And Allah says:
وَمِنكُمْ مَّن يُرَدُّ إِلَى أَرْذَلِ الْعُمُرِ لِكَيْلاَ يَعْلَمَ مِن بَعْدِ عِلْمٍ شَيْئاً
And among you there is he who is brought back to the miserable old age, so that he knows nothing after having known. (22:5)
The meaning here -- and Allah knows best -- is that Allah is telling us that this world is transient and will come to an end, it is not eternal and lasting.
Allah says:
... أَفَلَا يَعْقِلُونَ ﴿٦٨﴾
Will they not then understand!
meaning, will they not think about how they were created, then they become gray-haired, then they become old and senile, so that they may know that they were created for another world that is not transient and will not pass away, and from which there is no way out, which is the Hereafter.
