Why are people so arrogant about an age?

I cannot stand it when adults and teenagers, especially teenagers get so arrogant about an age. Once they are in 7th grade they talk so condescending to the 6th graders. The 10th grade acts condescending towards the 9th graders. I don't know how many times I have heard a 11th grader say, "aww look they are so small," even though they are like 1 inch shorter. Then adults, they act like the are so much wiser. Is