Earlier quoted context omitted.
"As you age there is no amount of exercise that will allow you to maintain the amount you weigh in you 20's or 30's." This seems overly general and anecdotally total BS. I still eat what I like just like I did in my 20s/30s. It's just that what I like is generally lots of vegetables and sufficient protein, with occasional vices slipped in because YOLO(tm). That said, without the exercise, I gain about a pound per wee…
My health prof in college said people gain weight because they don't maintain their muscle as they age...muscle requires 30 calories/pound just to maintain.
Anyway, the thirty calorie figure doesn't pass the smell test - men average about seventy three pounds of lean muscle mass, so at thirty calories per would be 2190 calories per day even if you ignore all activity and every other metabolic process. Your average sedentary middle aged guy is going to gain weight if he eats more than about 1500 calories per day.