In Cold Blood, Dick has been married twice, both with sixteen year olds. What do the brides reveal to the reader about Dick?

From his choice of sixteen year old brides, we can infer a few different things about Dick. First, you're correct, he seems to like younger women.... particularly teenage girls. Dick's interest in young girls illustrates his need to be in charge, as well as to seem ultra masculine and powerful. Young girls are easier led and more easily controlled. He takes advantage of their youth and inexperience with men.