I don't understand how author reasons that in his graph the "calories/day" for fat guy will be increasing with age, and for "lean" guy they will be decreasing.

I don't understand how author reasons that in his graph the "calories/day" for fat guy will be increasing with age, and for "lean" guy they will be decreasing. OK, so if metabolic rate is slowing with age, I would guess the "fat" guy's graph would not be so steep, or actually would be a level line (eating the same amounts of food), while for "lean" guy the line will be somewhat downward sloping (eating less the older he gets).