I had it when I was 14. Strangely, I don't remember feeling particularly bad with just the appendicitis. Then it ruptured, and let me tell ya...I was VIOLENTLY ill, in the bed just shaking. A day and a half later I felt much better, and even went to the beach the day after that. I was visiting family in CA at the time, and drove back to Washington a day or two after that. The day after getting home, I became super sick again. Went in, and my appendix had burst (probably the fist time I was sick), made my lymph nodes swell to the size of grapefruits, and that ruptured my intestines. The second sickness was the onset of gangrene. I came close to dying a couple of times. I spent two weeks in the hospital, the rest of the summer in bed at home, lost about 1/3 of my weight, and stopped growing until my early 20's. I also had stomach issues from the scarring which now, in my mid 30's, are only recently no longer an issue.
But yeah...the early appendicitis...I didn't even really notice it. I was just a dumb kid though and didn't know what to look for or when to be worried.