Interesting thread, thank you OP.
I’m approaching 50, and very slight. I run very competitively (or did before lockdown), and train every day. I love it, so it’s easy for me to do, I’d be useless at sowing, or even something equally active, but that I didn’t love. If you don’t love something, you won’t do it I’d imagine.
For the last 3 years I’ve also weight trained. Or rather, done a lot of core work and gone to the gym 2 times a week. Initially to prevent injury, but now, I see the benefit in of itself at my age, and surprisingly, I love it.
We were brought up with a healthy attitude to food. We didn’t have huge portions and didn’t have to clear our plates (unless my DM thought we were avoiding sprouts, or whatever). DH’s family similar, plus used to spend hours over a meal at the table afterwards, chatting etc.
DC are adults now, but home due to lockdown. We still sit at the table, unless it’s a night when we are all doing stuff, then one of us will have bunged something in the slow cooker.
One thing I notice incidentally, is that DS is like me, doesn’t eat unless hungry, and usually reaches for the fruit bowl or makes cheese and crackers. Dd and DH have far sweeter tooth’s and snack more, mainly on the less healthy stuff. While still slim due to genetics, they are not slight, like DS and myself. I wonder if this is nature or nurture?
Of all of the above, I suspect never seeing in childhood anything other than healthy attitudes to food, and no diets, aside from genes, probably the biggest factor.
I’m lucky too not to have anything physical, or medication etc that precludes exercise.
The one thing I was given though was Ribena and I had so many fillings as a kid. Sugar!