"Well, what do you think about him being younger then the rest? Do you like it or not? I sure don't!"
One thing to consider about a group is that somebody always has to be the youngest. Would you be equally, more so, or perhaps less displeased if Littlefoot was the youngest? Having a youngest member is unavoidable, even with twins.
"I have no problem with it at all. We don't know how much time passed after the hatching of the others anyway. Chomper's being a lot younger than the others (in addition to the minor inconvenience of his being a sharptooth) was not a problem either."
Chomper by species is naturally the most threatening character, so be making him so young, they dull down his image as a meat eater with cuteness. Look at the InuYasha anime. Fox demons in anime were usually fearsome- looking creatures like so:
http://farm3.static.flickr.com/2365/197516..._d2c775d1f0.jpg. So in that show, to make him more likable with the image built up of his kind, the writers made him very young and cute:
http://images1.wikia.nocookie.net/__cb2011...b/b5/Shippo.jpg.
When trying to entertain kids or even young adults, writers off dull down built up images of things to make them more likable, more sympathetic, more relatable, and more comfortable with. “Hm, we want a sharptooth to be in their group, but mostly the viewers have seen fierce ones trying to eat them!” “Ah, I know! Let’s make a kid for them, and we’ll amplify his lack of ferocity by making him cute by him being the youngest of them all, basically the innocence of the group.” If he was the oldest, I can pretty much guarantee that the episodes would be rather different than they are now. It isn't to me so much that there is no problem with it, but rather that the show would not be as we know it today. It's the same with Spike. If he was not the oldest, in theory, he would not have become part of the GoF, as he was found due to his hatching, and his discovery had factors from all four of the others. So if even one member of the GoF was younger than him, we might not even have a Spike. So I have no problem with an age that justifies one's existence.