hello!
I believe that there no rules and people get it no matter how old are them. For example, I know one girl, she is 29 years old and recently she was diagnosed with acute appendicitis, so as you see, no more rules when it comes to diseases like this.
I was reading some researchers in the USA and I found out that one in 20 people will get appendicitis. Even if a lot of us do believe that we are safe, it seems that we are not because appendicitis can strike at any age.
Sure, it is rare under the age 2 and most common between ages 10 and 30. So your sister is in that category, just like that girl I know.
I am 60 years old and just returned from the hospital, have come through surgery for acute appendicitis. Another strange enigma is that I had no rise in white blood cell count, but did have most of the symptoms. I also have a very wise doctor.I am doing great!
I am 58 years old and just home from hospital after having a gangrenous appendix removed. Two nurse friends have remarked that I'm "too old" to get appendicitus, but .... apparently not! Very painful, wouldn't wish it on my worst enemy, and because of my age, I am likely headed for a longer and more difficult recovery than some one younger, but that's life.