I was diagnosed at age 29, this year. My older brother has asthma, one of my uncles also has asthma, and my paternal grandfather had diabetes (I was told T2, but he was on insulin so I'm not sure). My aunt has Grave's Disease, an autoimmune thyroid problem. There is a strong history of ovarian cancer in my mom's side of the family. Because of my brother's asthma I was raised in a fairly sterile environment, although that was many years ago.
I was really interested to read that diabetes could be brewing in your body for 7 years before you are diagnosed, because I feel strongly like there was something wrong with me for a long time before I was diagnosed. I lacked energy, and chalked it up to depression. Three years before I was diagnosed, I did a VERY stressful university course and it took it's toll on my body. Near the end of that year, my cousin, whom I am close to, was diagnosed with leukemia. My body and emotions were hit hard by the news. Anyway, I finished the course and moved towns to do a very stressful job (if I had known at the time I wouldn't have taken it!). My office was also very dark and damp, and I'm sure has plenty of mould in it. I started on an SSRI. I gained a lot of weight over this period, and was sick very often due to sinus allergies. There were times when I visited my doctor once a month to try to clear it up, but nothing ever worked for long. At the end of that year my gran was diagnosed with terminal ovarianan cancer, and I enrolled to do a part-time Masters degree. My allergies continued to plague me, and then the doctor put me on prednisone. By the end of the week of taking it, my vision was blurred, I was exhausted, drinking excessively, struggling to breath normally and urinating so much I developed an infection.
While I think the prednisone and the allergies were the latest triggers, I can also see a family history of cancers and autoimmune disease.