Is it true that the Mahdi is born? I'm wondering about this question for a long time, I asked my Islamic teacher at my Islamic school, but it seems there is little information.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Ahl al-Sunnah wa al-Jamah (adherent of Sunnah) believe that Imam al-Mahdi will appear in the last period of this world. He will be from the descendants of the Prophet (Sallallahu Alaihi wa Sallam) Muhammad (Sallallahu Alaihi wa Sallam). He will fill the world with justice as it was filled with unjustice. There are many Ahadith proving this fact. For example the Hadith that the Prophet (Sallallahu Alaihi wa Sallam) said: "If it does not remain for this world but one day, surely Allah will send a man from my family who will fill the earth with justice after it has been filled with injustice and aggression" . [ Abu Dawood ]
However, when he will appear? Whether has he appeared or not? Where he will appear? No one knows the answer of such questions. So, we should entrust to Allah the knowledge of such matters.
Allah knows best.