Imam Al-Mahdi (AS) is in occultation according to Allah's order, waiting for suitable time to come to the public life to establish the world government of peace and justice.
Allah (SWT) only knows when He will permit Imam Al-Mahdi (AS) to come to the public life to establish that great aim which is in fact the aim of all the Prophets and Infallible Imams.
Imam Al-Mahdi (AS) is living on this earth and he guides the scholars and helps people in the way that Allah (SWT) guides him. He is away from publicity and even those whom he helps do not know him. They will recognize him after his re appearance.