The theory of “birth order” is not founded in the New Age but is the product of Alfred Adler (1870 – 1937), a Viennese psychologist who was considered, along with Sigmund Freud, to be one of the founding fathers of psychoanalysis.

One of Adler’s most noteworthy achievements is the development of the theory of the inferiority complex; but his theory on birth order never reached the same level of acceptance. To this day it lacks significant empirical evidence to support it.

The birth order theory purports to identify strengths and weaknesses in a person’s psychological makeup according to the order in which they were born. Adler believed that in a three-child family, first-born children, although initially in the most favorable position, were the most likely to suffer neuroses later in life due to the sudden loss of their parents’ undivided attention to a second sibling. After this point, they become burdened by a sense of responsibility for younger siblings. Second or “middle” siblings were the most likely to feel attention-starved due to being sandwiched between the first born and the youngest who would likely be overindulged and thus grow up with a lack of social empathy.

There are many variations on the above theories, but most of them are considered to be myths in the field of psychology.

Nevertheless, birth order studies are ongoing, such as those described in this article. For example, a 2012 study by University of Georgia psychologist Alan E. Stewart considered to be the most definitive work on the theory and research on birth order distinguished between “actual” birth order (ABO) and “psychological” birth order (PBO) meaning the position a person perceives themselves to have in a family.

The conclusion reached was that “Your perceived niche in your family plays a larger role in influencing the adult you’ve become than the actual timing of your birth.”

Birth order theory makes interesting reading, but that's about as far as it goes. It is neither New Age nor scientifically well-founded.