That's the age people will be able to buy the game by themselves. I honestly have no idea how well most shops enforce this age limit (if at all), but otherwise kids could and do still get the game from their parents.

Besides, other TW games are rated differently. Both my MTW and STW copy are rated 12+. If fans of these games venture in the MTW2 boards, they'd encounter language that's not appropriate for their age.

Besides, I'm sure most people here like the fact that patrons are expected to express themselves without using foul language.