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)

[لِكَىْ لاَ يَعْلَمَ بَعْدَ عِلْمٍ شَيْئًا]

(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.''

[وَاللَّهُ فَضَّلَ بَعْضَكُمْ عَلَى بَعْضٍ فِى الْرِّزْقِ فَمَا الَّذِينَ فُضِّلُواْ بِرَآدِّى رِزْقِهِمْ عَلَى مَا مَلَكَتْ أَيْمَـنُهُمْ فَهُمْ فِيهِ سَوَآءٌ أَفَبِنِعْمَةِ اللَّهِ يَجْحَدُونَ ]

(71. And Allah honored some of you over others with wealth and properties. Then, those who are so honored will by no means hand over their wealth and properties to those (captives of war) whom their right hands possess, so that they may be equal with them in that. Do they then deny the favor of Allah)