man even the kids on Family Guy changed ages once every couple of years, heck Meg is an adult now. The Kids on South Park moved up a grade and age wise Stan has been ten as of 2011 (but then again I'm probably the only person that is thinking right now that they are pushing more focus on Randy than the kids)
I actually do have a good theory as to why they won't age (even if the network won't let them age) my theory is that Mr.Burns made a longevity ray that would keep him alive forever but somehow this effects the whole town as well, and if that ray gets destroyed, everyone dies! granted that sounds like something that could have been done in the simpsons comics but Bongo got shut down last year