The hadith reveals that Hazrat Mahdi (as) will also have young followers. Due to the fact that most people in the End Times will be unbelievers and the frightening nature of the corruption of the antichrist, the majority of Hazrat Mahdi’s (as) followers will be young.
Most of these followers [of Hazrat Mahdi (as)] ARE YOUNG while the old are very few. (Bihar al-Anwar, vol: 52, p. 334)
The followers of many prophets in history have been young people. Allah reveals this in verse 83 of Surah Yunus:
No one believed in Moses except for a few of his people out of fear that Pharaoh, and the elders, would persecute them. Pharaoh was high and mighty in the land. He was one of the profligate. (Surah Yunus, 83)