“One day, gender will not matter in government,” he wrote. “Meanwhile pioneers.”

Of the five women in leading ministerial positions in the coalition, four are under 35. Asked about her age after it was announced that she would be prime minister, Ms. Marin reiterated what she has said numerous times: Age doesn’t matter.

“I have not actually ever thought about my age or my gender,” she said, according to the national news outlet YLE. “I think of the reasons I got into politics and those things for which we have won the trust of the electorate.”

She outlined her political ethos in a letter to fellow party members ahead of the decision on Sunday, asking for their support. She noted that she had benefited from the welfare state throughout her life, especially during “difficult times” and said that ensuring its strength was a priority for her.

“I got to live a safe childhood, have an education and pursue my dreams,” she wrote. “Enabling it for everyone has driven me into politics.”

While Ms. Marin’s new role has drawn attention globally, Anne Holli, a political science professor at the University of Helsinki, said it was unsurprising, in Finland where women’s representation in Parliament has been strong for decades.

In the 1983 election, women held 30 percent of the seats. By the 2007 election, they made up more than 40 percent of lawmakers, and they make up 47 percent in the parliamentary term that began this year.