Eating garbage food will also make you feel bad, no matter your age.

I used to eat junk food a lot in undergrad, and since I developed a tolerance/got used to feeling shitty I didn’t realize how much it impacted me day-to-day. Once I started grad school I shifted to healthier eating (real, unprocessed food), and quickly noticed how much better I felt, how much more energy I had. Now if I eat fast food (rarely) I always feed like crap right after the meal.