It may seem a stupid rule, but it's a rule for valid reasons.
We've had poor experiences with people under 18 in the past, and whilst wishing not to tar everyone with the same brush, it's something we need to address.
At the end of the day, if you can present yourself on an application properly, precisely, with excellent grammar and communicative skills, I see no reason why you shouldn't be in with a chance of being hired.
However, many under 18's have educational commitments, or otherwise, and cannot spare the time for the hours that OFPEC requires, and this is different from someone at say, 22 in Uni or College, in that they are at a more responsible level of control with regards to how their time is spent, but to have someone whom is sacrificing their studying hours to review Missions or Addons or whatever else, is simply not on.
We make that call on the best interests of the individuals concerened, and it shall not change as a standpoint rule, but we shall see how it goes if someone would like to apply.