Watching it online like you did, or via the DVDs, probably makes it much easier to lose track of that. Seeing it on TV, when it's being interrupted every ten minutes for commercials for Bratz and Barbie and Happy Meals, and sandwiched between much more obvious kiddy fare like Spongebob, really hammers home the intended demographic.

There are quite a few older Avatar fans out there who got sucked in because their younger siblings...or their own children...were watching the show first.