What causes this type of skinniness

[Mealworm]

I feel like there are two types of skinniness, and I can't quite explain it. In example 1, it's the type of skinny I experienced—it's common among people under 18. It wasn't due to a lack of exercise because I was always very active. Yet, there was no definition, and my forearms and biceps were the same size. Example 2, on the other hand, is the type of skinny many people over 18 have—it's bigger, with more definition, but still falls under the category of 'very' skinny. However, I don't even perceive it that way, people said R.R was frail, but I disagree. What causes example 1 of skinniness, and why do more people under 18 fall into that category?

 

[High Cortisol Chad]

As people get older, up to about 30 yo, they usually gain a bit of muscle just existing not even working out but it's not much. Then after about 40 yo there begins a slow decline.

 

[lilith]

muscle