So, what is the scientific reason I cannot eat as much as I did, 30 years ago?

Metabolism slows as we age. When your metabolism slows down, your appetite is not as strong. Kids and teenagers are growing, and they are typically pretty active, so they often require a lot of fuel.
My kids are 11 and they can eat twice as much as me, haha.

Simple reason: loss of muscle mass.

when you are a teen you are also still growing, your bones are getting stronger and setting into place and your skin cells become replaced more easily. Your metabolism also slows as you get older and you dont move around as fast as you did 30 years ago (e.g. running up the stairs all the time) even if you are very active now you will most likely be moving at a slightly slower pace. My mum is much slower than me with walking speed, running up and down stairs and she also doesn’t go out dancing all night. not to say she isn’t very active, she is probably more active than me in the sense of doing specific exercise. It is just that your body slows down and so do you and I doubt you are out dancing til 6am anymore…

You were also more active then. Think what your day was like 30 years ago. It sure didn’t include sitting at a PC for hours.

Nathans Hot Dog eating champion Kobiashi wrote a paper explaining that when you are thin, your stomach can make room for more food, but when you are heavy, the body fat won’t.

I could eat as much of anything I wanted until I was about 22 and I was 15 pounds under my ideal weight. Then I put on a pound a year and got to my ideal weight when I was 30. It was only then that I had to watch what I ate a little. I was lucky, hyper-active, and had to exercise a lot to get to and from classes. Your metabolism changes as you age.