My brother, 21 yo, is like this. He’s ~180 cm tall and has never weighted more than 61 kg (during the 2020 covid lockdown, he made a big deal out of this since it was the first time he passed the 60 kg mark; after the lockdown was over he immediately dropped back to his perpetual weight of ~55kg). I live with him so I know his eating habits: he eats all the time, whatever he wants, whenever, how much he wants and simply doesn’t put on any weight. Is very energetic, no sick look / demeanour. It’s just fascinating to me cause I’m not like that at all, I have to watch what / how much / when I eat.
I was 61kg at 187cm from 15 to 30. I was eating 6000 calories a day to maintain that weight. Then I gained 22kg over the next three years and finally felt healthy for the first time. I had all the same health problems as anorexics while eating like an Olympic athlete.
It was just genetics. Multiple family members were like this but not as bad as I had it. Now I have a nephew who has it just as bad as I did. He just has to wait another decade or so to achieve normalcy.
Similar metrics here. I finally managed to break 70Kg at 35.