first off, you should not transfer from kog or syn to XG just because you turn 18. there are many mature people in both kog and syn. kog and syn are not for the immature, but they simply don't have the age limit that Xg has. i honestly don't think that transfering on ur 18th birthday is a very mature thing to do... think about it, you would be leaving behind your clan in kog or syn, just because you want to be in XGC, the 18+ clan. XGC is not the big boys clan, it is simply a place for mature gamers. transfers are a good thing if you are having issues with whatever clan you are currently in, or you truely do not enjoy the experience you are getting in kog or syn...
as for the specific age, someone else already nailed it on the head... 18 is the legal age (at least in the states, and i believe most countries now a days) that you are an adult. if the government believes that you are old enough to grab a gun and fight and die for your country, than you are def. old enough to be a part of a mature gaming community. i realize that M rated games allow 17 and older, but we feel that 18 is a better indicator. we also understand that the system is not perfect in that there are 16 yr olds that are as mature as a 25 yr old, and that there are 40 yr olds that act like they are 12. but an age limit is the best way that we can make a definite requirement. there was even the idea of a 21 yr old limit when we first made XG, but the idea was thrown out due to many reasons, 1 being that the age limit for alcohol is 18 in most every country except US...
did i answer your question or is there still something unclear? let me know, i'll do my best