Great question! Here in Illinois, I strongly encourage most landowners to grow their black walnut to at least 24" DBH (diameter at breast height measured 4.5' above ground level). The larger you grow your black walnut trees, the more valuable they become. You will be money ahead if you get a professional forester to assist you in the selection and marking of your timber for sale, that way you don't harvest your timber prematurely!
There are, however, several instances where a professional forester will mark black walnut trees under 24" DBH: (1) Crown- or basal-damaged walnut trees; (2) Field-edge walnut trees that are susceptible to epicormic branching; and (3) Stand improvement cutting / overstocked conditions that warrant selective removal of underperforming black walnut trees.
Again, your best bet is to always get an unbiased, professional forester involved to make sure you're not harvesting black walnut trees before their optimum economic / financial maturity!