How might we explain that? How about arguing that there was less racial discrimination in 1948, or back then black teens were more highly educated than white teens? Of course, such arguments would be nonsense. The fact of the matter is that while there was a minimum wage of 40 cents an hour prior to 1948, it had been essentially repealed by the post-World War II inflation; however, with successive increases in the minimum wage, black teen unemployment rose relative to white teens to where it has become permanently double that of white teens.
If the minimum wage law has these effects, then how does it pass political muster? The current Social Security debate over private accounts gives us a hint. In the political arena, you dump on people who can't dump back on you. Few politicians owe their office to the youth vote. Despite the "concern for the children" malarkey they spout, it's voting age adults to whom politicians are beholden. It turns out that adults benefit from the discriminatory effects of minimum wages, and older adults benefit from Social Security intergenerational transfers.