An 1866 statute still on the books prohibits the placing of a portrait of a living person on “United States currency and securities.” For a very long time, the U.S. Post Office Department properly interpreted that as applying to stamps. In more recent times, U.S. Postal Service authorities modified the interpretation to suggest that living people could be shown as long as they were not being honored. Then, the postmaster general announced a couple of years ago that in his view the statute does not apply to stamps at all, and the field was clear to put living people on U.S. stamps in any way the Postal Service sees fit.

Source: Linn's Stamp News