Why do you get diabetes from a young age
About diabeties because I am 20 but I... - Diabetes & Hypert...
There are lots of different types of diabetes but there are two major mechanisms - either something stops you being able to produce insulin (type 1) or something goes wrong with the way you respond to insulin (Type 2).
The first mechanism involves damage to the pancreas which is the organ that produces insulin. This is generally an auto-immune response but people who have damaged or deformed pancreas for other reasons will not produce enough insulin to regulate sugar levels in your blood.
The second mechanism is often a response to consistently elevated blood sugar levels eg because of poor diet/lifestyle - eg too many refined carbohydrates (sugars) and not enough exercise. There are genetic components but they are difficult to assess because they are 'latent' genes which means that they need other environmental factors to activating them and it is difficult to know exactly what factors are needed for a given genetic variant to become active in a way that leads to diabetes.
Given the age at which your diabetes started it is likely to be the first mechanism that is at work for you.
There are a lot of sites where you can find out more about how diabetes works
These are links to a couple