i want to do the db quest, is there a minimum level

No...If you ask the keeper of any http://www.uesp.net/wiki/Skyrim:Inns about rumors, speak to the children at http://www.uesp.net/wiki/Skyrim:Honorhall_Orphanage in Riften, or eavesdrop on conversations or speak with people—particularly guards—in Windhelm, you will likely hear of a boy named http://www.uesp.net/wiki/Skyrim:Aventus_Aretino from level one.

http://www.uesp.net/wiki/Main_Page

cool

If you mean the Dark Brotherhood questline, there's no minimum level.