Hobi you've played long enough that you already know the answer to this.
Your level does not dictate when you will be able to enter a mainline world. You must complete the quests to advance in the game, and young Wizards will progress at different rates depending on their play style.
Once you have completed all your required assignments in Wizard City, Merle Ambrose will ask you to investigate Krokotopia.
Once you have been given that quest, you can access the Spiral door by entering Bartleby the World Tree.
After you’ve completed Krokotopia, you can move on to Marleybone, then MooShu, then Dragonspyre, then Celestia.
Grizzleheim is the only exception as it is a level based world in conjunction with the quest requirements. You can visit Grizzleheim at level 5, but cannot return until level 20.
