20s to 40s metabolism isn't that different. 4-5 pints is nearly 1000 calories, and basically nobody has a 1000 cal decline in base expenditure from 20 to 40.
A bigger change, generally, from someone in college and someone in their 40s is their activity levels. Even just considering the amount of walking most folks do on a college campus is a huge difference, compared to someone that gets in their car and drives to and from work.