Causes

Because Mikey is so rare being diagnosed at 6 months old, I've done extensive research on this. I'll try to sum up some of what I've learned, so bear with me if I make any mistakes or any gross over-generalizations.

With cats, in the veterinary world, there's Type 1 and Type 2. These are in no way comparable to Type 1 and Type 2 diabetes in humans. All it means is if the cat is permanently on insulin (Type 1) or if the cat eventually achieves remission or has a very low dose because the pancreas seems to be working, at least partially (Type 2). For example, canines are considered to usually have Type 1 (because they're usually unable to achieve remission) whereas felines are considered to usually have Type 2 (because they're sometimes able to achieve remission). Other than that, there isn't really too much additional research or distinguishing characteristics of different types of diabetes in felines.

In reality, there's more like 2 1/2 types of feline diabetes. I'll be using the human terminology so it's easier to understand.

• Type 1 - extremely rare/non-existent, most likely because the cat dies before diagnosis or never makes it past the first few weeks/months of life. (Mikey is pretty much an original.) Young age at diagnosis and no other co-morbid or pre-existing problems that could cause it and no signs of potential remission (i.e. dose never decreases and/or only increases).
• Type 2a - One of the most common types diagnosed in usually older cats (over around 9 years of age). Usually caused by a genetic pre-disposition being exacerbated by a long-term diet high in carbs (not good for the more carb-sensitive obligate carnivores that they are). High chance of remission if caught and treated early (within the first 6 months). After that time-frame, remission is still possible but might take longer or not happen at all.
• Type 2b (sometimes called Type 3) - Usually "transient" depending on the cause. Another common type diagnosed in kitties, usually over the age of 5 but it is also the most common type diagnosed in younger cats (>5 years). This one is an externally-induced type of diabetes, meaning something else is causing pancreatic damage or elevating BGs. This can be anything from infections, dental issues, CRF/CKD, IBD/IBS, heart disease, steroids, stress, pancreatitis, or other co-morbid conditions. Chances of remission depend on the underlying factor and the damage done to the pancreas (which isn't currently measurable so there's no way to know for sure). Some cats even fall in and out of remission (i.e. only when steroids are needed or whenever they have dental problems). Can also be exacerbated by a high carb diet.
• Type 2c (sometimes lumped in with Type 3) - Diabetes caused by a specific hyperglycemia-inducing problem, such as Cushing, Acromegaly, IAA, or EPI. This can hit cats of all ages. Usually identified by cats requiring an extremely high dose (more than 5u BID, even on wet, low carb food). Remission depends on the underlying cause. Mikey might in fact have one of these conditions, but he's never shown any symptoms and has never gone over 3u so I've never had him tested for any of these.

There are no real tests available (except for the high dose conditions) to tell you which type your cat has, but usually, you'll be able to tell if you know your cat's age and history.