The awaited Mahdi, Muhammad ibn Hasan (Peace be on him) was born during the lifetime of his father. He is in occultation, but is alive and will continue to live so long as the world lasts, for an infallible Imam must exist at all times. It is an agreed opinion of the Ummah that there could be no time when there would not be a master, either known and manifest or hidden and concealed. Moreover, Allah's grace is obligatory at all times. The Imam is His grace and so his existence is also obligatory.
His long life is not a far-fetched idea, for during the past ages many other persons such as Prophet Shu'ayb, Noah, Luqman, Khizr and Isa (Jesus) (Peace be on them) and Iblis (the Devil) and Dajjal (the Imposter) lived three thousand years or more. Moreover long life is possible, and whatever is possible, is within the power of Allah.
Imam Mahdi (Peace be On him) is not expected to have concealed himself of his own accord, for he is infallible and he cannot refrain from doing what is obligatory. Nor can be ordered by Allah to conceal himself, for Allah being Just and Judicious would not order him to do a wrong thing. Obviously it is improper for an Imam to hide himself from the eyes and to deprive the people from being benefited by his instructions. Therefore his occultation is due to the abundance of the enemies and the infidels and the paucity of the supporters.
The Mahdi must reappear, for the Holy Prophet has said: “Even if only one hour of the world was left, Allah would lengthen that hour until a man of my progeny would appear. His name would be the same as my name and his patronymic (kunyah) would be the same as my patronymic. He would fill the earth with justice and fair-play as it would be filled with tyranny and oppression. It would be obligatory for everybody to follow him.
Certain benefits continue to accrue from the Imam even during his occultation in the same way as the sun is beneficial even when it is under the cloud or a lamp is beneficial even when it is behind a curtain.