Despite the fact that Celiac Disease is a genetic condition, doctors believe that it is "kicked into gear" at some point in your life. Studies show that a stressful situation may bring it on, although there is a number of people who have active Celiac Disease from birth.
So basically, I spent my whole life living with Celiac Disease, and sometime in my mid-late teens, it kicked into gear. From that point on, my immune system started attacking my intestines when it encountered gluten, even though I was healthy before then.
It is estimated that most people wait 6-10 years between Celiac Disease acting up, and it being diagnosed.
This is another reason that so many people may be undiagnosed.