"Half Life 2 is a violent game with rampant death and blood. There is occasional obscenity, and perilous situations. The intensity of the situations, especially near the end of the game, can be somewhat overwhelming. But none of that goes beyond a PG-13 movie, or is much worse than Halo, which is also rated 'M', but commonly played by the 'T' crowd. I suggest parental discretion be used with this game when deciding on letting someone under 17 play - it is intense, and bloody, but nothing like Soldier of Fortune II or even Vampire the Masquerade: Bloodlines."
Not too convincing, but if you are allowed to watch PG-13 movies at your own whim then maybe they will let you play this game as well.