As of chapter 175, little is known of this situation.
However, it is known that Aladdin does not have the memories of his early childhood. Thus, he would not remember his rank as a Magi until he was reminded.
Not much is known about Aladdin's past, but he has found out who he is and where he came from thanks to Solomon and Solomon's Wisdom. The first memories he has are being in the Room of Fortitude with Ugo, where he was taught everything he knows by him.
— Aladdin, Magi Wiki
That said, there are some speculations as to why this may be the case:
- Because he is likely a descendant of Solomon, or some reincarnation of him, the memories may have been hidden to prevent some potential goal.
- It may also be the case that Aladdin is not actually a Magi, but something else that appears as a Magi. Thus, he may never have known his purpose prior to losing his memories, either.
However, none of these are confirmed nor backed by substantial proof, so take them with a few (read: several) grains of salt.