The guy was at least 19. In 2012, that was the minimum age for a male LDS missionary.
LDS Church lowers age requirement for missionary service
President Thomas S. Monson announced that effective immediately, young men may begin their full-time missionary service following their graduation from high school, even if they are only 18 at the time. And young women, who have not been eligible for full-time missionary service until age 21, may now begin their service at age 19.
“I am not suggesting that all young men will — or should — serve at this earlier age,” President Monson said. Rather, he said, the option is now available based on individual circumstances and the recommendation of their local church leaders.