It could be the nature of the immune response. In children and adolescents, the body produces white blood cells extremely quickly to adapt to a wide variety of infections - that means they have an extremely effective immune system that may not stop them getting ill in the first place but they will shake off the illness very quickly.
In adults, there is essentially an army of tailored white blood cells which mean you have more of a natural immunity. But I think adults will produce novel white blood cells at a slower rate than children do. So assuming there was an immune system trigger for your diabetes, what probably has happened is that you've had good immunity throughout your life, then encountered something new, your body has made new white blood cells but at quite a slow rate, which means your islet cells had more of a chance to grow back - thus meaning it's more a gradual destruction process rather than the abrupt impact you'd expect from a younger immune system.
Just speculating, the above could be nonsense but hopefully the logic's consistent.