Some of the traditions concerning Imam-e-Mahdi (a.t.f.s.) describe that time would not turn him old, and he would continue to look young. In one of the traditions it has come down as follows:
“Aba Salte Haravi I asked Imam Reza (a.s.): ‘What is the sign of your Qaem (so that one can recognize him)? Hazrat Reza (a.s.) replied: Amongst the signs is that he would be old age, but would look young in appearance. Anyone looking at him would imagine him to be forty or even less. Amongst his other sign is that time would never turn him old until death meets him.” 
In another tradition, Imam Hasan-ibn-Ali (a.s.) says:
“When Mahdi (a.t.f.s.) will rise, the people would deny him because he would approach them with the face of a youth, while they would imagine him to possess an aged face.” 
Again in another tradition come down in the book ‘Ghaibat-e-Tusi’, Imam (a.t.f.s.) resembles prophet Jonah (a.s.) by possessing a youthful face.
It’s worthy to mention that all the fortunate ones who have met Hazrat, during minor or major concealment, have seen him with a youthful face; and appearing as a man of forty. We pray to Allah to make us fortunate too, Inshallah.
 Muntakhab-ul-Athar: 284, tradition 2 from Kamaluddin.
 Yanabi’ul-Mawaddah: research of Sayyed Ali Jamal Ashraf al-Husaini 3:393 (chapter 94, tradition 39)
 Muntakhab-ul-Athar: 285, tradition 6