they seem like different sizes, not ages.  i suggest you look at some actual references for all ages.  maybe pick a child star who's grown up so you have references at the three different ages. or even a child you know.  because they all look very adult (i'd say 28-30 something), the second looks just shorter and the third just petite.  you have to remember, it's not just chest size, and most 7-year olds are pretty much indistinguishable gender wise (imho)  if you don't look at hair and clothes.  younger kids are different proportions.  leg length, hand size, heads do scale some (try to put on a hat made for a kindergartner), weight changes, and even how joints look change.  young girls and even some adolescents tend to have knobbier knees, longer legs, bigger hands and feet, etc.  in fact, those are part of the characteristics built into anime females to make them seem younger.