Everyone at one point has preconceived thoughts about the near-and-or-just-over underage, and let me tell ya', they aren't always positive ones.
I think the reason the 12-15 year olds get flammed is because of the overwhelming majority of in-person 12-15 year olds they know that tend to act completely childish. THUS, making their actions and views towards them a lot less respectful.
However, it's just a problem that everyone has to face today - Stereotypes, racism, prejudice, etc...
As long as we have Freedom of Speech, they won't stop, and even our rights of expression were taken away, people would still find ways to insult anything.
Just have to learn to live with it.