During the events of Half-Life: Opposing Force, what is the age of Adrian Shephard, the protagonist?

In the events of Half-Life: Opposing Force, Adrian Shephard, the protagonist, is referred to as a 22-year-old marine corporal. This is established in the game's storyline, which follows Shephard's mission to quell the alien outbreak at the Black Mesa Research Facility. The specific age of 22 is likely chosen to depict Shephard as a capable, yet relatively young, military operative who is thrust into a high-stakes and critical situation, allowing players to relate to his character as a skilled but inexperienced soldier.