Board Thread:Tapped Out Help/@comment-173.75.246.3-20130710213811/@comment-14714013-20130816215414

That doesn't surprise me. I was at level 24 when the Muntz quests began.

To further explain, these quests doesn't start immediately upon reaching level 21. Rather, you must have attained a minimum level of 21 and also completed the Town Hall quest before they will become available. Once the latter of the two requirements is achieved, so the quests will be activated.