Imam Mahdi's appearance as the New Khaleefah:

Having gained the right to appoint the new Khaleefah, they will nominate a member of their army, a man from The Holy Prophet's family, for this position. This man will be The Mahdi, who had also joined the ranks of this army when they entered Arabia.
He will not accept this post and will fleet from Madinah Munawwarah to Makkah Mukarramah. However, his people will be convinced that this is the only man for, the job. And so consequently, a group of people from Makkah will look for him in The Holy City and find him making tawaaf. They will force him to accept the post of Khilafat. Imam Mahdi will reluctantly agree to this offer. They will actually lead him by the hand towards the Hajare Aswad, where people will then formally pledge their allegiance to him as the Khaleefah. This pledge will take place in front of The Holy Ka 'ba, between the Hajare Aswad and the Maqaame Ibraheem, during or just after the days of Hajj.
At the time of this Bay 'at a voice will call out from the heavens: "This is the appointed Khaleefah of Allah, the Mahdi!"
Initially the number of people to take the bay 'at pledge of allegiance at his hands will be 313, which is the number of Sahaba that took part in the battle of Badr. He will be forty years old at the time of his appointment.
Among the major signs of Qiyaamat is the advent of Imam Mahdi (Alayhis salaam). Rasulullah (Sallallahu alayhi wasallam) predicted the coming of Imam Mahdi towards the end of this worldly existence, i.e. just before The Day of Judgement.
Hazrat Abu Hurairah (radiyallahu anhu) narrates that Rasulullah (Sallallahu alayhi wasallam)said:
"If only one day is left for the end of the world, Allah will lengthen that day until a man from my family, whose name will be the same as mine, rules the world." (Tirmizi Shareef)
This indicates the emphasis, which The Holy Messenger of Allah Sallallahu alayhi wasallam laid on the coming of Imam Mahdi.
This booklet contains all the details about the life and advent of Imam Mahdi, and all these facts are derived from authentic Ahadith on this topic.
There is much speculation nowadays on whether Imam Mahdi has already been born and if so, what would be his present age, and when the impending appearance. Some Ulama have claimed that he is already born.
However, Muslims should realise that mere speculation does not represent concrete evidence upon which any firm and clear-cut ruling could be based. Hence, the best course of action is to believe that Imam Mahdi will appear' before Qiyaamat, but exactly when, only Allah knows. The date and time of his arrival we leave to The Knowledge of Allah. However, Rasulullah (Sallallahu alayhi wasallam) did give certain signs, which will herald the arrival of this great Imam. TheAhadith on this issue leave no room for rejecting or even doubting the coming of Imam Mahdi. Neither do the facts leave us in the dark as to recognizing this great Imam when he eventually does arrive.