A step-dame (stepmother) and dowager (widow holding property for her deceased husband) would make their son (or stepson) wait until they die to inherit all of the land or money.
Theseus is saying that he is tired of waiting for his wedding day to Hippolyta, comparing himself to that 'young man' waiting for his dowager or stepdame to die because to him, Hippolyta is a great fortune.
