According to another thread about an AB that went to a littles get together announced on fetlife, apparently some little's don't even know AB's exist. It was a surprise to me to hear, but I guess this AB had to tell them all about the AB lifestyle.
So yeah, like dogboy said though, little's can be really anywhere from a baby age, to the age of a grown child around 8 +/-. Basically, saying AB is kinda just narrowing it down a bit more, to the group of us who like to be a younger age, or at least the babyish aspect of wearing diapers and being taken care of.