I didn't become symptomatic until I turned 28. This is typical; some people are symptomatic as soon as they begin eating solid food and one of my coworkers didn't become symptomatic until she turned 58.
Symptoms vary but they all trace back to the small intestine losing its ability to absorb nutrients. Some people gain weight, others lose, others get anemic, etc. I started getting severe, roving abdominal pains that never went away. I personally lost 30 lbs in the 2 months after my symptoms started before I finally got an inkling about what was going on (doctors were clueless and useless--I had an upper GI scan done, an ultrasound, two CT scans, and numerous blood tests for various diseases and cancers). I know a guy who at the age of 2 was back to his birth weight before doctors figured it out. I know another woman who went from 120 lbs to 80 lbs and lost all of her hair before the doctors woke up.
After taking stock of my symptoms and having tried dairy, soy, and nut-free diets, I looked up some information on WebMD and saw that celiac disease matched a lot of what was happening to me. As a last resort I went on a gluten-free diet and immediately felt better. I finally got a doctor to order the correct blood tests (specific antibody counts spike with the onset of celiac disease). Once it was confirmed I was able to get the diet straightened out and stopped losing weight. My constant fatigue and depression/irritability went away too. Now I'm just crotchety.
I had an upper endoscopy done (small intestine biopsy) done to confirm what was going on. For that I had to go back on the gluten diet for a while. The pain and weightloss started up immediately. Now whenever I have the craving for pizza all I have to do is remember the hours in bed, clutching my stomach and being curled up in the fetal position.