The hadiths mention how old the Mahdi will be when he embarks upon his task, when people will recognize him, and when they will see and follow his activities.
He will be sent when he is between 30 and 40. The Mahdi is one of my children. He is around 40. (al-Haythami, Al-Qawl al-Mukhtasar, p. 41)
He is 40 years old. According to another account he is between 30 and 40. (Barzanji, Al-Isha'ah, p. 16)
The Mahdi is one of my people. He is 40 years old. His face is like a bright star. (Fera Idu Fevaidi'l Fiqr)