Why do children have health issues, what causes that?

I'm writing about child obesity and need to describe my social group which is children. I need to state why they have health issues what causes it ... could it be there age or because there just young. Lets say for example i was doing old people instead that would be easy as i could say old people have health issues simply they are old they have reduced immunity , reduced mobility, diet because of housing conditions etc.... But I'm struggling to start my description on children what could i write about them???Why do children have health issues, what causes that?99% of the time it has to do with the **** foods their parents feed them and lack of exercise. Most kids these days would rather be on the computer instead of going outside to play. Some children may have an under active thyroid also.