Full answer (longevity)
The objection here is the impossibility of such a long life. Let us examine it closely, the word impossibility here (like any other truth) is relative. It has meaning only in relation to some person, place and time. What is impossible for one person need not be so for the others. Then what is impossible in one place may be quite possible in another place. Again what is not possible at one time may be quite possible at another. There is no dearth of illustrations to prove how impossibility is a relative term. To journey across the ocean, to reach the bottom of the sea and to travel to the moon are practical possibilities. There are people who have accomplished these tasks in one way or another.” And yet they may be impossible for others due to varying reasons.
In other words, the possibility of a thing may be of three categories viz. factual possibility, scientific possibility and logical possibility.
By a scientific possibility we mean that there may be certain things which may not be practicable in the present circumstances but there exists no scientific reason to justify the denial of their practicability in favourable circumstances and the scientific trends indicate that they will be feasible sooner or later. For example there is no denying that there is a scientific possibility that man can travel to Venus. There is only a degree of difference between man’s landing on the moon and his landing on Venus, however it is scientifically impossible to land on the sun which is virtually a huge furnace blazing at the highest imaginable degree of temperature.
By a logical possibility we mean that, on the basis of self-evident and rational laws, reason does not regard a thing impossible. For example, it is logically impossible to divide 3 apples equally into 2 parts without cutting one of them is self-evident that, three being an odd number, it is not divisible into two whole numbers.”
Thus, we know that the scope of the logical possibility is wider than that of the scientific possibility and the scope of the scientific possibility is wider than that of the practical possibility. There is no doubt that a person's remaining alive for thousands of years is not logically impossible, for there is nothing irrational or self-contradictory about it. Life itself does not imply the sense of quick death.
Admittedly, such a long life is not as practical as descending to the bottom of the sea or ascending to the moon. Even those who have all the modern facilities at their disposal and are the keenest to continue to live cannot have more than the normal span of life.
Death under the scientific scope
In a nutshell, it is logically and scientifically possible to prolong life, though it is still practically impossible to do so. Anyhow, science is endeavouring to make it practical.
If we consider the question of the Mahdi's age in this light there appears to be nothing strange or surprising about it, for it has been proved that such a long life is logically and scientifically possible and the scientists are working to turn its possibility into a reality. All that appears to be surprising is that the Mahdi has attained such a long life before the scientists have been able to turn its theoretical possibility into a practical one. This phenomenon can be compared to the discovery of a cure for cancer or brain haemorrhage before science can make such a discovery.
If the question is how Islam, which planned the age of the Awaited Saviour, could anticipate science in this field the answer is simple. This is not the only field wherein Islam has anticipated science. The Islamic Shari'ah as a whole anticipated the scientific movement and the natural development of human thinking by several centuries. Islam had already presented, for practical application, the laws which science has taken hundreds of years to discover.
It has propounded doctrines, the wisdom of which has been corroborated by science only recently. It disclosed such secrets of the universe which none could think of at that time and the truth of which was later confirmed by science. If we believe in this, then it is not too much for Allah, the Exalted, to anticipate science in planning the age of the Mahdi. We have talked of only those aspects of anticipation which we can see directly. We can add other instances about which Qur'an has told us.
Here we have talked of only those aspects of anticipation by Islam vis-à-vis science which we can observe directly. It is, however, possible for us to add some other instances of anticipation about matters which it has not been possible for science to comprehend so far. For example, Qur'an tells us that the holy Prophet was carried one night from Masjid al-Haram (at Mecca) to Masjid al-Aqsa (at Jerusalem).
If we try to perceive the quality of this journey within the framework of natural laws it points to the application of laws which govern nature in this sense that science has not yet been able to understand them and it will take it hundreds of years more to specify their quality. The same Divine Knowledge, which granted the holy Prophet this high speed long before science could consider it to be possible, also granted long life to his last Divinely appointed successor, long before it could be achieved by science.