At first, Ms. Cilley, a Democrat, said she was sure Ms. Levesque had to be mistaken. When she realized that Ms. Levesque was right, she said she was astonished — and thought that changing the law to raise the age would be an easy lift.

“How could anybody defend the practice of 13-year-olds getting married?” she said.

But a bill Ms. Cilley championed, which would have raised the minimum age for marriage, ran into a wall of opposition from her Republican colleagues. Among critics of the idea was Representative David Bates, the lawmaker who dismissed Ms. Levesque as “a minor doing a Girl Scout project.” (Mr. Bates, who did not respond to a message left at his home, did not run for re-election this year.)

In March 2017, Republicans killed the bill, using a procedural move intended to keep it from being considered again for two years.

“It didn’t make me want to stop — it just made me want to push forward,” Ms. Levesque said. This year, with Ms. Levesque continuing to press the issue, Ms. Cilley and several of her colleagues passed a trio of bills which raised the minimum marriage age to 16 and gave judges more guidance about how to consider petitions for marriage licenses from people 17 and younger.

In the meantime, Ms. Levesque graduated from high school and went off to study photography at the New Hampshire Institute of Art, before deciding that a career in photography was not for her, and coming home. “I didn’t want to do wedding photos,” she said.

Two seats in the State House of Representatives for Barrington would soon be open, and local Democrats urged Ms. Levesque to run. She said she had hesitated at first. She worried about balancing political office with college classes and with her commitments as a leader of her own Girl Scout troop. Also, she didn’t know how she would get to Concord; she doesn’t have a driver’s license.

On the next-to-last day that she could file papers to run, she did.

In Concord, Ms. Levesque will certainly stand out. As of 2015, the average age in the New Hampshire State Legislature was 66, making it the oldest state legislature in the country, according to the National Conference of State Legislatures. One reason is that the State Legislature is essentially a volunteer job, with the legislators being paid only $100 per year plus mileage.