Eating too much of fast food, with a lot of bad fats and high salt content
Now a days, kids have computers, video game consoles and cell phones. They tend to be too sedentary. They do not go out and play like we used to as kids.
Some kids and adolescents have a problem with body image and are either bulemic or anorexic.
Children now a days are overweight or obese, and some of them have even developed Childhood Obesity. I have written hubs on both of these if you want to check them out.
I think children/adolescents like to experiment with drugs such as tobacco, alcohol and illegal drugs. Those can have serious complications later in life if they continue to use them.