But there are bad foods (processed foods, often full of sugar and unhealthy fats, and many takeaways and ready meals etc) and good foods (chicken, fish, eggs, (plus limited red meat and cheese) yoghurt, olive oil, lots of fruit, beans and legumes, all kinds of vegs, including potatoes , sweetcorn etc etc) , wholegrain pasta, wholegrain carbs of many different kinds, nuts, dark chocolate etc.
For active young people, nothing wrong with the occasional pudding , either. In areas associated with the famous healthy Mediterranean Diet (way of eatig, not slimming regime) www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/mediterranean-diet/art-20047801 traditionally people had really fancy sweet things like cake once a week, after a large lunch on Sundays. Or else for occasional parties. Otherwise, fresh fruit/yoghurt for afters.
I would have thought teenage years would be the ideal time to discuss this. Carefully, of course. NO fad diets, lots of tasty things to eat. Storing up good health for their adult future.
Diabetic is VERY different, of course.