TL is simply out to protect themselves, not to mention these kids, and for good reason. Let me put this into perspective. Granted this example is very rare and not likely to happen within the TL games, that fact remains that it COULD, and that possibility is MORE than enough reason for TL to maintain their position on this.
Myspace sued for not protecting minors
I'll just sum it up (you can find the horrible details in that link) and just say that a 14 year old girl was taken advatage of by a 19 year old adult who she met on Myspace. He faces 20 years in jail, she experienced one of the most traumatic experiences any person could, and myspace is being sued for $30 million.
It's just a horrible situation for everybody involved, and THAT is why TL does not let under 14's play, and 14-18's only with parental supervision.