I haven't read the other answers. My 17 yo dd has an athletic build. When she us in a sport all is well, when not....so, we began walking and got her a gym membership. I instituted sweets for the house only on "S" days by saying it was about me and my inability to avoid. I keep veggies and fruits cut and ready in the fridge. I never, ever, ever suggested any changes were about her. It's either health or the family helping me. I don't ask or monitor her at all. I think that would make her rebel if only unconsciously.