Will the Mahdi be born or just come from Heaven?

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?