The Mahdi will come as a sign that Hour is getting closer. We have also been told that he will be from the Family of the Prophet (saw) as he will be related to Fatima. I have heard that he will be here for about 7-8 years. Now Is that 7-8 years that he will rule or 7-8 years of his entire life? Will he be born on Earth or will he just appear as a grown man? Have we been told any of this?
Imam al-Mahdi will be a righteous man from among the descendants of Prophet Muhammad (peace and blessings of Allah be upon him), who will appear at the end of time, through whom Allah will set the affairs of the mankind straight, and will fill the earth with fairness and justice just as it was filled with wrongdoing and oppression.
Umm Salamah said: "I heard the Messenger of Allah (Peace & Blessings of Allah be upon Him) say: The Mahdi is of my lineage and family, from the descendants of Fatimah."
Sunan Abu Dawud, 11/373; Sunan Ibn Maajah. 2/1368. Al-Albaani said that it is saheeh in Saheeh al-Jaami, 6734
Ali ibn Abu Talib (may Allah be pleased with him) said: "The Messenger of Allah (Peace & Blessings of Allah be upon Him) said: The Mahdi is one of us, a member of my family. Allah will guide him in a single night."
(Musnad Ahmad, 2/58, hadeeth 645, edited by Ahmad al-Shaakir, who said: its isnaad is saheeh; Sunan Ibn Maajah, 2/1367. This hadeeth was also classed as saheeh by al-Albaani in Saheeh al-Jaami al-Sagheer, 6735).
Ibn Katheer said: "This means that Allaah will forgive him, grant him help, inspire him and guide him, when he was not like this before."
(Al-Nihaayah, al-Fitan wal-Malaahim, 1/29; edited by Taha Zayni).
So based on the above fact that Allah s.w.t will forgive him and help him in a single night proves that he will not come from the heaven and rather be here. As for the view that he exists at present and that he has been born, this is merely a claim that is made by some people, but there is no evidence for it. It is most likely that it is a myth. The ahaadeeth that have been narrated about the Mahdi do not mention where he will be born or when, and they do not tell us to look for him or to bring him up, rather they say that Allah will prepare him in a single night and that the Muslims will swear allegiance to him as their caliph and will be united behind him.
And in my opinion Muslims should not wait for the emergence of Imam Mahdi, rather they have to strive hard and devote their efforts to support Islam, and to do whatever they can for the religion. They should not hope and wait for the emergence of Imam Mahdi or anyone else, rather they correct themselves, their family and the people around them, so that when they meet Allah s.w.t they meet Him having done their best.
Allah Azza wa jal knows best.