Hello! I'm not a medical professional, just the parent of a kid with diabetes. Yes, type 1 diabetes is typically diagnosed in children, hence the name 'juvenile diabetes'. But adults do develop the disease. My brother was diagnosed at age 30, in fact. Recently I've heard of a lady in her 70s who was diagnosed with type 1. It is not frequent, but it does happen.
There is no known cause for diabetes, so I can't say what causes it. Wish we could figure it out, and soon.
Modern thoughts as to the cause is that most true type 1 folks have an autoimmune disorder that, when triggered, causes the body's immune system to go amok and not only attack and kill off the triggering virus, but also to proceed to attack and kill off the body's insulin-producing cells. The result is the death of the pancreas' insulin-producing abilities and diabetes follows. Most people develop diabetes as children, which is when we tend to get sick most often with viruses. But we do hear fairly often of people diagnosed in late teens through early 30's, too, and I have wondered if this is because in our college years we tend to be exposed to lots of viruses because we are so social during these years. And it is thought that diabetes can happen years after the onset of the triggering virus. In cases of identical twins, supposedly if one twin is diagnosed as diabetic, only 50 percent of the other twins are eventually diagnosed also as diabetic. Even though the genetic makeup is identical and both have the genetic immune system weakness, only 50 percent of the time does a virus actually trigger the pancreas attack. Fascinating, I think.