His saying:

[ثُمَّ نُخْرِجُكُمْ طِفْلاً]

(then We bring you out as infants,) means, weak in his body, hearing, sight, senses, stamina and mind. Then Allah gives him strength, gradually and causes his parents to treat him with tender kindness night and day. Allah says:

[ثُمَّ لِتَـبْلُغُواْ أَشُدَّكُـمْ]

(then (give you growth) that you may reach your age of full strength.) meaning, his strength increases until he reaches the vitality and handsomeness of youth.

[وَمِنكُمْ مَّن يُتَوَفَّى]

(And among you there is he who dies,) means, when he is young and strong.

[وَمِنكُم مَّن يُرَدُّ إِلَى أَرْذَلِ الْعُمُرِ]

(and among you there is he who is brought back to the miserable old age,) meaning advanced old age with its weakness in mind and body, in steady decline in comprehension, and disability to grasp. As Allah says:

[لِكَيْلاَ يَعْلَمَ مِن بَعْدِ عِلْمٍ شَيْئاً]

(so that he knows nothing after having known.)

[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 (you) weakness and grey hair. He creates what He wills. And it is He Who is the All-Knowing, the All-Powerful.) [30:54]