Comment by icegreentea2

So the results show that on average, metabolism is stable from 20 years onward. That means that there actually is a drop from 18 to 25 (it's there for men at least when you look at Fig 2). Now think about where most people will be anchoring their perception of "adult" from (it's going to be some value in that range). That means that there will absolutely be plenty of people who experience a real decline in metabolism in the years following their "personal adult threshold".

Now, that's a pretty small drop, so definitely the other life transition (probably starting an office job, living alone) will probably play an even larger part to any weight related changes. But even given a hypothetical case where someone kept the exact same activity level, the amount of metabolism decline over that period is probably small enough to delay the onset of any noticeable body composition change (like order of 10 pounds) for a few years, adding to the mid to late 20s experience.