Mostly to assist American viewers: the "Gretna Green" referred to many times in the episode is a village in Scotland, literally just over the border along one of the only well maintained roads of the time. When the story takes place, England had laws in place preventing anyone under 21 from marrying without parental consent, however Scotland had no restrictions on anyone over the age of 14, and furthermore the ceremony only needed to be witnessed by a set number of individuals; no ordained priest was required.
Florence Hunt is the only teenaged teenager in the cast. All of the other younger female character actors are in their mid-twenties to mid-thirties.