I've always seen it, through my opinion and from how other people identify and so forth:
AB: "baby ages" can be a bit all over the place, 0-3 almost always but some will include 4 or 5, some won't. Generally I'd say AB is about high degree of dependence on the parent, use of "babyish" items (sippy cups, pacifiers, and so on), pastels/bright colors, usually at least some diaper wearing.
Little: 5-8/9ish? Not entirely sure as again it gets messy quick, usually people I've seen who identify as little are more around 4, 5, 6, sometimes bit older. Generally a lot of overlap with AB, not quite as dependent and zeems to somewhat often have no involvement of diapers.
Middle: This one, should be easy considering, but I have no idea where the hell middle goes, I couldn't even really give a good idea of what "middles" would do. Guess go with the between point from little to teen.
Teen: 14+
Ages usually seem to mostly have to do with the level of care, independence, and then stuff like clothing. Which I realize then goes against the whole age range thing to begin with (since someone could like to be 14 but treated as a baby and so forth).
So getting back to the question suppose it depends on how we want to look at AB age, if it is simply the age you want to play while being babied then in theory the oldest age could be 100+! If it is about the dependence and all the stuff mentioned a bit ago, personally I'd say maybe 5 for the oldest