On the basis of the science of physiology the human body is composed of Billions of cells and tissues. With the passage of time, the cells get old and worn out and wither away and young cells take their place. This is how the schedule of life goes on.
That thing which worn the man out and stop the cells from performing their activity and work and brings the gift of death to man are the harmful microbes which enter the human body from various ways and start a combat against the cells and annihilate them.
The knowledge of medicine (prevention and cure of disease) is a witness to it that if man commands the perfect knowledge and acquaintance with the condition of body, diets and the perilous and harmful substances and completely looks after his health and is careful about his food his life will prolong. He will not meet an early approaching old age.
From this point of view the scholars and scientists have been able to prolong the lives of a few animals to many times their (natural) length, through experiments. In this way, man can, by virtue of vast knowledge and respecting the rules and regulations of health, live for a number of centuries.
A scholar must search for many long years to reach and lay his hand upon a secret of the knowledge, to get acquainted with only a piece of it. But Imam Mahdi all the stocks of whose knowledge are bestowed upon him by the God does not need to traverse all that long way and he can travel the length of centuries in one moment and reach the result.
In this way, it would not be anything difficult, from the scientific point of view that he (Imam [A.S.]) by the vast knowledge, which has been gifted to him by the God, may lead his life for hundreds of years with health and safety of his youth and old age and wearing off does not approach him.
On the other hand, the length of the life of Imam Mahdi is not more surprising and amazing than the extinguishing of the fire (of Nimrod) for Abraham, the bifurcating and division of the river Nile for Musa/Moses to cross over and turning of a pole into a serpent, which are the show and manifestation of the power of God.