For instance, would there have been a difference in me eating a grain-heavy diet at age 12 and eating a similar diet at age 20? At 12 I had a flat stomach, but at 20 I had a little gut (I was skinnyfat).
Basically, my question is: Was my body just "handling" the problematic diet back when I was 12 and kind of lost a little control over it at age 20? Or does the insulin restistance only happen to obese people or older people? Or was there something else going on here?
IR appears to happen naturally to some extent as part of the aging process, even in hunter-gatherer populations or those avoiding the "neolithic agents of disease". Hard to tell with current societies so horribly skewed by generations of eating crap (what the academics call 'confounding factors'). Environment may play a part -- I think stress, especially chronic, has been shown to increase IR but don't have a link handy. Getting fat (visceral fat, adipose tissue) can be a sign of IR, but plenty of lean people develop it as well. Some people eat junk all their lives and never get it.
Part of the problem, and this may end up too meta, is that conventional definitions of just what IR is seem awfully circular. Even after distinguishing between muscle and liver insulin resistance, there's obviously a lot more going on than just simple carbohydrate quantity, like the quality (or lack thereof) in the neolithic agents like grains (especially refined and/or unfermented).
My first memory of reactive hypoglycemia was at about age 14, maybe before. When we were kids, we used to walk a few blocks to the local Circle-K and get either a 44oz soda or an Icee and some kind of candy of our choice. We'd do this for a substantial portion of the summer. My diet resembled that of a hummingbird during the rest of the year as well. Standard SAD carb/grain intake on top of that. Sure we were outside riding our bikes or committing various forms of mischief until it got dark every day or playing soccer etc. but it was simply an insuperable fructose load. Even once I cut out sugar years ago I was still eating tons of bananas every day and having a somewhat similar but slightly muted set of symptoms.
Fortunately it's all gone away, though I may have nuked some pancreatic beta cells or something along the way.
It occurs several different ways but the most common is 15 to years of central leptin resistance then leads to muscle leptin resistance then prolonged elevated HbA1c and insulin resistance. Once this occurs adrenal resistance tends to very quickly follow and Neolithic diseases appear with regularity.
In light of how well low-carb treats insulin resistance in some of the population, while having virtually no effect in others....
There is probably relatively little risk of insulin resistance occurring in a person who is not overweight and/or hypoglycemic. The mechanisms involved seem to lately revolve around refined sugar starting the cycle and refined carb exasperating it. But this is only occurring when these items are taken in and accumulate instead of being burned for energy.
Activity and growth at 12 would likely limit any insulin resistance issues if excess weight was not present, although it is unlikely you could judge how you should have looked precisely when 12 on your own.