Hypertrophic pyloric stenosis is rarely seen after 3 months of age. There have been only six well documented cases reported previously. No one theory adequately explains the etiology of all cases.
Konvolinka CW, Wermuth CR. Hypertrophic Pyloric Stenosis in Older Infants. Am J Dis Child. 1971;122(1):76-79. doi:10.1001/archpedi.1971.02110010112023