- Appendicitis is an inflammation of the appendix, a small worm-like pouch attached to the large bowel. It can happen at any age but most cases are between 8 and 25 years of age. It is rarely seen in children aged under two years. For young people, appendicitis is probably the most common cause of stomach pain requiring emergency surgery.www.netdoctor.co.uk/diseases/facts/appendictitis.htm