Here's a great explanation :The first reason, is a LEGAL matter. There are laws about marketing and advertising to minors on the internet. COPPA - Children's Online Privacy Protection Act http://www.coppa.org/coppa.htm read ^those details and you may understand why Yahoo! MUST (by law) delete underage users
But more importantly (in my opinion) are a zillion safety reasons. Kids don't realize that all the little tidbits of information that they post here (in questions and in answers) CAN be used by predators and endanger them in REAL LIFE.
The "friends" that kids (and even naive teens & adults) *think* that they are communicating with online... could actually be your worst enemy, a neighborhood predator, a kidnapper, or worse.
I've seen innocent_looking Polls, that ask questions that are actually common security questions for password reset/recovery systems. So hackers can use these answers to compromise you and your computer.
Adults can pretend to be anything (or anyone) that they think you'll fall for. (ie: "Hey Im 13 too! email me!") in order to interact in private. ^can be used as a ploy to gain private communications with an underage child. And Yahoo! does NOT want kids to be in private communications with people that could be unsafe.
Because, there are questions on here that some of us older kids, and adults shouldn't be reading, so why should younger ones read it to. Just like, you're supposed to be 18 to have a Facebook or a Myspace.