genetics change every 7 years, it may catch up to you RSX.
What? Have you ever taken a biology class? Where the f did you hear this? Genetics do not change...genes are encoded in your DNA...invariable save the occasional mutation, but with all the genetic redundancy, that is negligible.
Cells replace themselves every 7 years or so (depending on the body part, of course), but they keep the genetic information.
Now, what does happen and is relevant is that as you age your metabolism decreases. It's to the degree of about 2% every ten years.
But to the person who eats poorly and is skinny regardless, you should stop that behavior now. Some people have fat cells which are reluctant to grow or multiply, but you still have to worry about the health problems commonly associated with obesity, such as high blood pressure and cholesterol. In fact, because you do not get fat, you should be especially careful and monitor this as a hidden risk.
As for this 7 year old, even if she does have a health problem causing her to be this way, someone should have intervened done long ago.