Type 2 can be reversed. But some people are so prone to it, reversal only lasts so long as they manage their life style. Others can consume much greater amounts of carbs so long as they're being healthy, after becoming not diabetic. Often the condition is dependent on how long the person's body was out of control and if the damage is beyond repair at a rate the body can cooperate with.
Insulin sensitivity can be reversed, but for example nerve damage can be so extensive that they're not coming back particularly at the persons age.
Reducing the amount of AGE's in the body helps significantly, but if a persons glycation is on a runaway process it's not easy. And many people's cells are somewhat defunct from a lack of healthy fats so the respond poorly to insulin, and to make matters worse autophagy isn't happening even when they do consume healthy fats because many things interfere with it, such as blood sugar levels and stuff that comes with getting old.
Some people eat pretty healthy, or as healthy as people around them, and gain weight and become diabetic. They may have a disposition or it may be as simple they're eating not only the wrong items for them, but nothing that nurtures their body despite the fact they're told they're eating healthy. Wheat of any kind would be an excellent example because it has a protein in it that binds to insulin receptors, and for some people it bind really damn well. They cut it and they lose weight and feel great despite not being a celiac; and of course they're publicly ridiculed because people that don't understand microflora or cellular binding of proteins tell them it's a bad fad since gluten free foods don't have anymore typical vitamin and minerals in them than regular food (many processed ones have more fat and sugar, too).
Type 1 is awful. It's not well known but reducing things that encourage an imbalance with the immune system for these people will help. You may have noticed some T1's are perfectly healthy except they take insulin, and others die before they're 30... Lifestyle can ruin them just like anyone else, or improve them. It just doesn't yet stop the autoimmune function (although research is not far off).