You're confusing the two.
Type 2 is the most common, it used to be called adult-onset diabetes (and type 1 was juvenile diabetes), however, as the prevalence of type 2 in younger individuals has risen, the names have been changed. Essentially, types 1 and 2 diabetes are 2 different diseases with the same name, because the name diabetes mellitus in Latin describes the symptoms, not the cause.
In type 2 diabetes an individual's body has either become desensitized to their own insulin or cannot produce enough insulin to control blood glucose -- often it's a bit of both. This is usually caused by poor diet and lack of exercise, and obesity (the reason more people are developing it younger and at younger ages). However, there is also a strong genetic factor, and though it's less common, individuals who lead very healthy lifestyles and are at a healthy weight can also develop type 2 diabetes, usually later in life.
Type 2 diabetes is not necessarily treated with insulin injections. Some people can successfully control type 2 with diet and exercise. Some require oral medication to lower blood sugar (this is not insulin), and some do take insulin injections. Type 2 diabetics have functioning islet cells and produce both the hormones insulin and amylin.
Type 1 diabetes (what I have) is an autoimmune disease, that has no known specific cause but does have a strong genetic factor and some known environmental factors. An individual may develop type 1 at any age, though most often it develops in the toddler years, at puberty, or in the late teens/early twenties. It is rare for it to develop in individuals after their early 20s. Generally a viral infection serves as the anticedent, it is possible that diet and weight may be factors, though research is limited and there is not an obvious link as there is in type 2. (I was a very physically fit child at the time I developed diabetes.) The body's immune system destroys the insulin-producing islet cells of the pancreas, so that the individual must inject insulin to survive (type 1 diabetics do not produce the hormone amilyn either, though the use of amilyn in the treatment of type 1 is still experimental and began in humans only a few years ago). Type 1 diabetics must test blood glucose more often than type 2 diabetics, because we are at much greater risk of diabetic ketoacidosis caused by extreme high blood sugar, and hypoglycemia. Type 1 diabetics, because they produce no natural insulin, must be even more careful with their diets than type 2 diabetics, as carbohydrates will raise their glucose higher and faster than in a type 2 individual. A type 1 diabetic who does not take care of himself can also develop type 2 diabetes, and have both types at once.
To sum up, both types are treated similarly with glucose monitoring, and diet and exercise, however, although the symptoms are the same, the causes of types 1 and 2 diabetes are very different. Type 1 diabetics are insulin-dependent, and type 2 diabetics are not.
