If it's an epigastric hernia (which is most likely, considering it's caused you no issues and hasn't been operated on) then it most likely developed during the very early foetal stage. Epigastric hernias are largely a cosmetic nuisance as it's just fat from the abdominal wall (not intestinal tissue) that's poking through. However, sometimes that fat can get trapped and cause the area to become sore and red, but the body prunes the trapped fat off and reabsorbs it, unlike an abdominal hernia where trapped tissue can quickly become a life-threatening situation due to sepsis.