[briHass]

As with everything, it depends largely on your genetics and natural inclination to becoming insulin resistant. Insulin resistance (IR) can occur in normal-BMI individuals. This is typically accompanied (caused?) by a large amount of visceral fat (internal, around your organs), which isn't what we typically see when we look at an overweight individual. IR is really just the first step to full type 2 diabetes.

Exercise and youth can negate much of a bad diet, but it will likely catch up to you eventually. Better markers than your weight might be HDL (should be high), triglyceride levels (should be low), and fasting blood sugar. If those are optimal, you're probably doing fine, but there isn't really a case to be made that you should continue to eat poor food choices if you can avoid it.