To be fair though, Rickon was not fleshed out very well in the books either. Mostly because he WAS a baby. He just followed Bran around the castle crying for his family (and making me concerned about that beast of his with the stupid name).

As far as the casting goes, I agree that the children had to be older both for the reasons you mentioned, and also because we'd never get the stellar performances we've seen from younger kids. And boy-oh-boy, I've been blown away by the performances from all the kids on the show.

Nope, you're not the only one; they've done a fairly terrible job of introducing him. (If I recall correctly, he's had exactly one substantial scene, when he was in the tombs with Bran and Osha, after Ned died.)

All the kids are older, but I think that's a good thing: I think the audience would have a hard time watching kids as young as the ones in the books go through what they do. (It's bad enough Sophie Turner—at 15—had to perform that near-rape scene on this week's episode, but I can't imagine it with a 12 or 13 year old actress.)