I watched 90 minutes of Mark Zuckerberg's interview with Joe Rogan. He mentioned that his wife encouraged him to train because it improved his personality and made him nicer to be around. My conclusion was that physical training is the most important aspect of life and impacts everything else. Donald Trump claims the only thing he does is golf, which is quite remarkable. Maybe Trump is an exception to the rule. He has never appeared fat to me, but eats fast food.
Obviously, if you're already succeeding at life by eating fast food, drinking soda, and not working out, by all means, continue your streak. But for a teenager, or young adult, I can think of no better piece of advice.