Diabetes in adult is known as type 2 Diabetes, maturity onset diabetes or non insulin dependent diabetes. This usually occurs after the age of 30 year. This type of diabetes occurs due to decreased secretion of insulin as well as decreased effect of insulin (already less produced) in body i.e. insulin resistance. Thus diabetes in adults occurs due to twin defect namely insufficient insulin production in the body as well as what ever insulin is being produced works less meaning insulin is less effective in lowering the blood sugar (i.e. less ineffective also known as insulin resistance).

This less insulin production & less insulin effectiveness may be due to following causes: 
1) Hereditary factors: There are two type of inherited defect in gene of the persons with diabetes as: 
      a) First one is defect in genes of insulin producing cells so that their insulin producing beta cell (in pancreatic gland) is not able to secrete enough insulin so that less insulin is produced i.e. insulin deficiency. 
      b) The second hereditary defect, there is inherited defect in insulin molecule or its working mechanism so that what ever insulin is being produced is not very effective in lowering the blood sugar level because either insulin itself is defective or their body organs are inherently resistant to secreted insulin action i.e. insulin resistance.
Thus glucose reaching in blood from intestine is not utilized by body & sugar concentration in blood keeps on increasing to higher than normal resulting in diabetes. The above mechanism is the cause of diabetes in vast majority of patients.
2) Many drugs as steroids, growth hormone, diuretics taken for longer times leads to decreased insulin secretion & makes insulin less effective causing drug induced diabetes.
3) Many toxins as insecticides found in many food items may damage insulin secreting glands.
4) Many hormone disorders, as excess of Cortisol hormone, growth hormone, hyper aldosteronism etc decreases insulin secretion & lead to diabetes.
5) Chronic alcohol use may lead to pancreatic damage leading to insulin deficiency & diabetes.
6) Diabetes during pregnancy occurs due to release of certain hormone during pregnancy which decreases the effect of insulin secreted from pancreas leading to diabetes. The diabetes which develops during pregnancy is called Gestational Diabetes.
7) Latent Diabetes
8) Borderline diabetes
9) Impaired Fasting Glucose
10) Impaired Glucose Tolerance

There are following types of diabetes in children: 
1) Type I (i.e. Insulin dependent diabetes mellitus): Type I diabetes is also known as Insulin dependent Diabetes. It occurs at young age often below 25 yr. of age due to destruction of insulin forming beta cells in pancreas by a particular type of viral infection. These viruses infect the pancreas & then damaging insulin forming beta cells leading to total deficiency of insulin hormone so that blood sugar rises. Such children are dependent on insulin injection for their survival. If they miss insulin injection for few days then their blood sugar level reaches very high & they may go in coma. Some times ketones appear in blood & urine (called as diabetic keto-acidosis). This may be sometimes life threatening also.
2.) Type II i.e. Non Insulin Diabetes of young (NIDDY): This usually occurs in obese children with positive family history. In such children diabetes is initially controlled on tablets, later insulin may be required by some.
3.) MRDM (Malnutrition related Diabetes
4) Fibrocalcific Pancreatic Diabetes (FCPD)
5) Drug induced: steroid induced
6) Auto-antibody induced.
7) Secondary diabetes
In Children after investigating the type of Diabetes appropriate treatment should be prescribed.