Your Wizard's level has nothing to do with when you enter Marleybone. Just like you had to complete all your required assignments in Wizard City to progress on to Krokotopia, you must now complete all your required assignments in Krokotopia to be invited in to Marleybone.
Once you have completed the quest "Temple Dweller" where you are called to face and defeat Krokopatra, you will be offered entry in to Marleybone.
Tired of "what level" just finish all of Proffesor Winthropt's quests. After that, Shalek The Wise will tell you to go to 4 places (2 are wich dungeons). He will be in the dungeons but General Khaba and Ako/Arena Master will tell you what to do. Then you will go to the Krokosphinx and all you have to fight are bosses (only elite you will fight are if they are with the bosses). Ghost Of Krokotept will tell you to defeat Krokopatra. Once you have defeated her dungeon, (yeah a dungeon you will get over it) speak with Sergeant Major Talbot, he gives you a spiral key to Marleybone to rescue the Krokonomican.