If I remember correctly, this topic has been done previously at least once. Try a search on these forums, and you might find more information readily available. (However, I realize that some of these threads may have been lost over time, so no offense intended.)
FWIW, I'm a last-born (4th child), and know many other pilots -- all who have had a successful flying career for 15-30 years -- who are not first-borns.
It's an interesting theory, but I'm not quite sure that the actual reality supports the initial perception.