“What causes someone to develop diabetes at an older age?”

What causes diabetes? Well, that is quite a question. You are young, age 26, with diabetes. Why? You have an unhealthy lifestyle and are overweight and have used up your insulin (your body has just run out of insulin), or you inherited a gene that causes it, or you unluckily developed an autoimmune disease that has antibodies that destroyed or are destroying the islet cells. That is a simple basic way of looking at it. A c-peptide will tell you if you make any insulin at all. Since I know little about you, I can't give you more in-depth reasons.