I know a sophomore who two years ago was in his second year of 12 & Under as an 8th grader, so this year as a sophomore he would qualify to wrestle 14 & Under. I think he was a September birthday and he started school early. Starting early is not common though, so there should not be a lot of sophomores in 14 & U. Most sophomores will be in 16 & U in Kansas.
As far as a 15 year old 7th grader, I am not sure if you really meant that. Most 7th graders in Kansas are in 12 & under since they do not turn 13 until September 1 or after. There are some 7th graders who turn 13 before September 1 and they have to wrestle in 14 & Under. These students either started school a little late or might have been held back a year for some reason. I believe you would find more in this group than in the group that started school early, so you should see more 7th graders than sophomores in 14&U.
The norm is 8th and 9th graders in 14&U, I would bet that over 90% of the wrestlers in 14 & Under are either 8th or 9th graders.