In children, indirect inguinal hernia is of congenital origin, and is due to incomplete obliteration of the processus vaginalis. No such cause applies to indirect hernia in middle age, since one cannot assume that a congenitally unobliterated peritoneal protrusion would remain quiescent during young vigorous adulthood only to become symptomatic in middle life.
An indirect hernia in this age group seems almost universally associated with a spermatic cord lipoma and/or other properitoneal fat adherent to the sac. Such fat tissue is easily deformable, and has almost fluid-like characteristics permitting it to insinuate itself into tissue planes. The internal ring is a channel through the transversalis fascia, and the gradual introduction of fat by internal propulsive forces can widen that pathway. Parietal peritoneum can enter the defect under repeated pressure from above, be dragged into the breach by its adherent fatty pseudopodium, or both.
In two studies of recurrent inguinal hernia,
ROSENBERG N. "Lipoma" of the Spermatic Cord: Potential Relationship to Indirect Inguinal Hernia in Adults. Arch Surg. 1979;114(4):549-550. doi:10.1001/archsurg.1979.01370280203038