It's actually nowhere near as simple as that, and it depends on how you define the "same activity".

Moving your body from one location to another at a particular speed results in a fairly static amount of work done (in the basic physics sense). So if the same you hops on a bicycle and cycles up the same hill in the same conditions with the only difference being that one of you has trained hard for the last 5 years and the other hasn't (but somehow your body mass has stayed the same) then you'll burn exactly the same amount of energy. Untrained you will find it much harder, but the energy burn will be roughly the same.

There are some things that can be different as you go from "untrained" to "trained", for example lighter people burn less energy moving themselves around than heavier people. Trained people tend to do activities harder/faster so the "same activity" could actually be a much harder activity despite it not feeling that way. Although if the activity involves travelling a set distance some of the extra effort involved in doing it faster is offset by the fact that it takes less time, so the difference between the two is not as large as you'd think.

The study says that TDEE is generally constant from 20 to 60. Multiple studies have shown that on average people do a terrible job of estimating their food intake ([1], [2] for example), especially when they aren't actively monitoring it. Meanwhile the person you're talking about (who isn't properly "OP") is claiming that they ate more in their 20s than they do today without corresponding health effects. This is not consistent with TFA nor with previous research.

1: https://www.nejm.org/doi/full/10.1056/nejm199212313272701

2: https://www.tandfonline.com/doi/abs/10.1080/08870446.2019.16...

(This thread is now hella old, but only look at comments every once and awhile, thus the late reply).

I'll just say the same thing I always say in these kinds of "who are you to deny science!?" replies. For everyone championing how hard, complex, and subtle weight loss is, there's a "dumb" gym bro just weighing him self every day and dialing back calories until the scale starts going in the right direction. It works every single time. 100% of the time.

I don't understand where does the excess basal energy expenditure go. Do persons like the OP when <25 y.o. produce significantly more heat (e.g. wearing only t-shirt and shorts even in winter, and "dying" in the summer)? Does the skinnier body mean way bigger heat losses, even when compared to bigger surface area after getting fat (i.e., way higher surface temperature)? (presumably yes, but to this extent?) The article states that the basal expenditure increases with "fat free mass", so someone fat but muscular will have this "overheating" problem even stronger?

N=1, but visceral fat starts building up sometime after age 40 anyway. About 52, in my case.