I think "big kid" is just the term used for these little ones to feel like they are big, and a little more is expected of them, we don't want to rush growing up just because they are taking on a little more like potty training and following the rules. "big kid" should be used loosely in the sense of trying to make a little one feel more responsible. My kids are still little kids to me and they are 9 and 11. now I have a nephew who is 16, I consider him a "big kid" def not a man or adult though!