Thanks to a tip from Miss Caladon Rae, I traveled to Oatmeal 15, a testing sim for Adult Land. Nearby sims are PG or Mature rated; number 15 is Adult rated, and provides a test for whether the Lab considers one an adult.
As I hit the sim barrier, I received a notice that said I could not access Adult content - but that I could change this in Preferences. Sure enough, in the new RC, there is a "Rating" setting. The default, stupidly, is for "PG and Mature." Setting it to "PG, Mature, and Adult" let me through.
Wheeee! "Ding! You are now free to move about the country," as the advertisements say. Now back to my sedate life in the 19th century.