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.