Dogs of any age can become diabetic, even puppies. However, it is much more common for older dogs to develop diabetes, especially if they are overweight.
Diabetes in dogs is almost always Type 1, or insulin-dependent, diabetes. This condition occurs when insulin-producing cells are attacked by the animal's own immune system or when hormones interfere with insulin production. Most dogs with diabetes develop it in middle or old age, much like humans, although humans that develop it in adulthood tend to have Type II.
Some dogs are born with genetic defects that can cause Type 1 diabetes from a very young age; others develop diabetes only after reaching adulthood and going into heat. The hormonal fluctuations of heat and pregnancy put intact female dogs at higher risk for diabetes. Juvenile diabetes runs in certain breeds; golden retrievers and keeshonds are especially prone to it.