In Man there is a Lesson
Allah says,
وَاللّهُ خَلَقَكُمْ ثُمَّ يَتَوَفَّاكُمْ وَمِنكُم مَّن يُرَدُّ إِلَى أَرْذَلِ الْعُمُرِ لِكَيْ لَا يَعْلَمَ بَعْدَ عِلْمٍ شَيْيًا إِنَّ اللّهَ عَلِيمٌ قَدِيرٌ
And Allah created you and then He will cause you to die; and among you there are some who are sent back to senility, so that they know nothing after having known (much). Truly, Allah is Knowing, capable of all things.
Allah tells us that He is controlling the affairs of His servants. He is the One Who created them out of nothing, then He will cause them to die. But there are some of them that He allows to grow old, which is a physical weakness, as Allah says;
اللَّهُ الَّذِى خَلَقَكُمْ مِّن ضَعْفٍ ثُمَّ جَعَلَ مِن بَعْدِ ضَعْفٍ قُوَّةٍ
Allah is He Who created you in (a state of) weakness, then gave you strength after weakness, then after strength gave (you) weakness. (30;54)
لِكَيْ لَا يَعْلَمَ بَعْدَ عِلْمٍ شَيْيًا
so that they know nothing after having known.
meaning, after he knew things, he will reach a stage where he knows nothing because of weakness of mind due to old age and senility.
Thus Al-Bukhari, when commenting on this Ayah, reported a narration from Anas bin Malik that the Messenger of Allah used to pray;
أَعُوذُ بِكَ مِنَ
الْبُخْلِ
وَالْكَسَلِ
وَالْهَرَمِ
وَأَرْذَلِ الْعُمُرِ
وَعَذَابِ الْقَبْرِ
وَفِتْنَةِ الدَّجَّالِ
وَفِتْنَةِ الْمَحْيَا وَالْمَمَات
I seek refuge with You from
miserliness,
laziness,
old age,
senility,
the punishment of the grave,
the Fitnah of the Dajjal and
the trials of life and death.
Zuhayr bin Abi Sulma said, in his famous Mu`allaqah;
"I became exhausted from the responsibilities of life. Whoever lives for eighty years, no wonder he is tired. I saw death hitting people like a crazed camel, and whoever it hit dies, but whoever is not hit lives until he grows old.