I haven't read any responses to this post yet.
However, speaking from experience, do not judge people based on what you think you know about them.
As any of you would see (from my progress pictures via the Members Pics section) - I used to be severely underweight until my early 20's ... well past my growth / maturation stage.
Regardless - I train my legs religously - and while my thighs (Quads, Hammies, Glutes) are responding very well ... my calves, of which are a known problem to many people, are not responding. In addition to this, my wrist and ankle circumferences / breadths, are dreadful - which you'd see from my progress pictures mentioned earlier.
Nothing infuriates me more, than someone saying to me: "why don't you train your legs?" , when I actually do so. Because, while my legs are growing, my calves in particular (which are the visible part when wearing shorts) are lagging behind the rest of my body; not due to lack of training, but due to the obvious fact that some of us grow differently, and certainly, calves are a known trouble-area for many people.
In short?. Don't pass judgement without knowing history first.