Praise be to Allah.
Allah, may He be glorified and exalted, says (interpretation of the meaning): “And Allah has created you and then He will cause you to die, and of you there are some who are sent back to senility, so that they know nothing after having known (much)” [an-Nahl 16:70].
And He, may He be exalted, says (interpretation of the meaning): “And among you there is he who dies (young), and among you there is he who is sent back to senility, so that he knows nothing after having known” [al-Hajj 22:5].
At-Tabari (may Allah have mercy on him) said:
Allah, may He be exalted, is saying: Allah created you, O people, and brought you into being, when you had been nothing, and it was not the gods whom you worship besides Him (who created you). Therefore worship the One Who created you, and no other, then He will take your souls in death.
And some of you will grow old, and end up reaching a stage of senility, which is the worst of stages. We only bring him back to senility so that he will once again become ignorant, as he was in infancy and childhood, so that he will not know anything after the knowledge that he had when he was young. All of that will be lost when he grows old, and will be forgotten, and he will not know anything of it, for it will escape his mind. So after having possessed the power of rational thinking, he will not understand anything.
As-Sa‘di (may Allah have mercy on him) said:
Here Allah, may He be exalted, tells us that it is He Who creates people and causes them to move through the stages of creation, one after another. Then after their lifespan has ended, He causes them to die, but He causes some to live until they are sent back to senility, i.e., the worst age, in which a person loses his outward and inward strength, even his power of reasoning, which is the essence of man. It grows weaker and weaker, until he forgets what he used to know, and his mind becomes like the mind of an infant. End quote.
Tafseer as-Sa‘di (p. 444)
Az-Zajjaaj (may Allah have mercy on him) said:
What is meant is that among you are some who grow very old, until they lose their power of reasoning, and after having known things they become ignorant. Allah does that in order to show you His might and power; just as He is able to cause a person to die or give him life, He is also able to cause him to move from knowledge to ignorance. End quote.
Zaad al-Maseer (2/571)
What is meant is that Allah, may He be exalted, causes people to move from ignorance to knowledge, then from knowledge to ignorance, and from weakness to strength, then from strength to weakness.
People vary in the extent of this ignorance to which they move. Some of them will become completely senile, so that they do not know anything at all; some of them will hardly know anything; and some of them will know a few things but ignorance and lack of knowledge is mostly the case for them.
What is meant by the words of Allah, may He be exalted, “so that he knows nothing after having known” is to highlight the fact that ignorance will prevail after he had knowledge, not to rule out knowledge altogether for everyone who reaches this stage, because saying that he will know nothing is not to be taken literally; rather it is by way of emphasis.
Hence Allah says, “and among you”, so as to indicate that not everyone who grows old and reaches old age will be sent back to senility; rather some of them will be sent back to senility so that they do not know anything after having known something, and some of them will grow old but Allah will protect their reasoning, hearing and sight.
Ibn Jaziyy (may Allah have mercy on him) said:
It does not mean that knowledge is ruled out altogether; rather it is a reference to having little knowledge and to forgetting a great deal. End quote
Tafseer Ibn Jaziyy (1/431)
Az-Raazi (may Allah have mercy on him) said:
If it is asked: How come Allah says: “so that he knows nothing after having known” [al-Hajj 22:5], when he may well know some things, like a child? Our response is: What is meant is that his power of reasoning will be taken away, so he will become as if he does not know anything, because such a thing may be described in the negative for the sake of emphasis. End quote.
Tafseer ar-Raazi (23/205)
Ibn ‘Ashoor (may Allah have mercy on him) said:
Hence there are levels of weakness of reason, depending on the degree of senility. It may go so far as being unable to learn anything new, but before that there are different levels of weakness of reason, such as forgetting things, mixing up information, and so on. End quote.
At-Tahreer wa’t-Tanweer (17/202)
These two verses do not mean that everyone who grows old will be in this state of “senility”; rather among those who grow old there will be some who are sent back to senility and will be in that state where they forget everything that they knew before.
Moreover, this forgetfulness and being sent back to senility are of varying degrees, and people vary in that regard. There are some who forget everything that they knew, and others who suffer to a lesser degree. This is something that is very well known.
And Allah knows best.