I am asking specifically about adults. I think I have a rather large rib cage and though I am not considered overweight, I would like to lose some weight and tone up. Will my bones change at all during the weight loss? I always wonder how people who lose a significant amount of weight once supported their mass on their seemingly small frames.

I lost 90 lbs, i'm in the young twenties and the only thing now is that my back hurts more but then again i do lift heavy things at work a ton. so....its not so big a diff. but ur bones r ur bones, they won't 'change'
The way not to do it is starve yourself, to start with. Just exercise and eat right. I did....lost 90 LBS because of it. Starving yourself won't help at all. Drink more water, eat less before bed, and don't eat for stress are the 3 things I follow and i'm going pretty well if i don't say myself. 90 LBS isn't anything like 5-10lbs so take my advice. If you want anymore details, feel free to email me. Just please be patient for ppl seem to email me for advice on this, I will reply, don't worry! ~Adia

I think it has more to muscle mass and fat rather than bone structure, rather than bones.

No, but the skeletal muscles do reduce in size.

the bones change stability and can result in osteoporosis