As salamu alaikum IMAM MAHDI will kmow who he is in his late teens.Anyone know anything about spirituality know the HAL comes first its no knowledge its just experince,then the MAQAM comes "its experince and BATIN knowledge"sooner or later in MAHDI case later.MAHDI will not know whonhe is from.hadiths he will know from his spiritual state his reality.The first yo make BA'A to mahdi want make ba'a fron reports of hqdiths it will be from the pure noor ALLAH places in their heart.