There is no specific reason as to why you need to be 16 to join the army but 18 to play the game. Some say that it is because the game is a bit tough and rough for a 16 year old to play. You only get to play at the age of 16 under a parent's consent.
The game is not age appropriate for youth but you actually have to be 18 to join the army too. It's wrong for youth to use dangerous adult weapons like bazookas and flick fire air force jets. But guns are okay for the age of 16. Just not army weapons.