I don't think we need another life stage either. As a mother of 4 kids, I can tell you there isn't much distinction and I find the whole tween idea silly. It's a short transition between about 10-12 where they stop playing with "little kid" toys and start doing things that are slightly less "childish". But major changes in activities and interests happen after they hit their teens.
The biggest gap is actually between toddlers and children. The age from about 3-6/7 is missing. But, as I said already, I don't want anymore life stages.