Sure they exist, but we don't want to talk about it. There is a strong motivation to be able to say that everyone is created equal, regardless of how uncontrollable factors affect you (birth order, height, race, gender). We don't want to kill someone's motivation to study for college just because they are a third child. But stereotypes exist for a reason. The hard part is separating hatred-based stereotypes from the stereotypes based on observations throughout history.

Regarding birth order, the "middle child" stereotype is prevalent in Western society. Did the middle child stereotype spring from some sort of effort by first-borns to keep their lower-ranked siblings in line (a la racism?)? I doubt it. The middle child stereotype probably came from hundreds of years of observations. It's a similar story for the first-born and last-born stereotypes- they weren't created out of any prejudice, but probably from hundreds of years of mothers chatting about their kids.

Like the article says, denying that the differences exist prevents us from ever finding out why (because people won't do research on something that doesn't exist). So let's acknowledge that some groups of people are different than other groups, and then respectfully do research to figure out why. The differences could be societal- perhaps first-borns are treated with more care by their parents, or they could be biological- perhaps the hormonal changes in a pregnant woman are different for each successive child, or they could be something else entirely- I had many of the same teachers as my older sister, and I often benefited from them having a pleasant experience with her, making my time in class much easier.

But anyway, the first step is acknowleding that a difference exists, and that you aren't somehow "birth order-ist" just because you acknowledge that first-born kids often outscore lower order kids on many metrics.