Shows how terrible my history on shipping is! To be fair, I didn't watch the show with fans back then, so I kind of assumed it all started with the Book 2 finale. I still think overall in the narrative (in terms of the structure and the screen time), I felt it was pretty obvious that it was always going to be Aang x Katara. I definitely can see why people thought Aang was too young. Their relationship (especially as Katara got older in Book 2 and 3) kind of felt more paternal (like she was his mother/sister)...so yeah.
Thing is though, for all the gripes people had with ATLA with regards to romance, I still say that was better than the crap we got in Korra. Because at least the romance knew to take a back seat and not get in the way of things.