Sunday I think there are some who do in their 20s and early 30s. Getting towards 40s I think far fewer, as that's when metabolism tends to catch up with you whatever your lifestyle. I think I mentioned upthread that I was one: I never ever thought about what went in my mouth, and I weighed under 8 stone. When I think about what I used to eat I find it unbelievable, but like I said, I've spent an inordinate amount of time wondering what changed, and although some of it may be age / metabolism, my weight gain very definitely started at the point I moved in with DH; I noticed it almost straight away and I didn't get round to really doing anything about it until it'd crept up to about a 10-11kg gain, because I was in shock at having to think about it at all.
The fact that I could so exactly pinpoint when it started makes it easier to analyse what I'm doing differently, and when I think about it, it really is just small changes which added up: as my friends and I got older, had more disposable income, and life got a bit more hectic, our social lives came to revolve around sitting and consuming. Before then, even though I feel like we ate and drank loads, it tended to be huddled around a picnic bench in a pub garden in winter coats, chain smoking and slugging wine, but then someone'd go and grab a few bags of crisps or a couple of bowls of chips, and then we'd drunkenly inhale some toast when we got in - there wasn't a real meal, and there was a lot of walking around either side. It was just a lot more active, being cold and shouting over each other and bobbing up and down the whole time. Compare that with what we started doing once we could afford it and were tired after a day at work: going to the pub meant finding somewhere with a warm dining room, sitting much more relaxedly, putting away a 3-course meal, and probably even more wine because it was with food. Or go to each others' houses and do the same. Get a taxi back and forth if we were feeling lazy or it was cold or an awkward journey instead of stomping between bus stops.
On top of that I started having wine with dinner most nights because I could keep it in the fridge without it getting pinched by unscrupulous housemates 
; again this puzzles me because I lived alone at university and did the same, but back then, I had to walk about 20-25 minutes to uni every day (and back, obviously) and thought nothing of "popping home to get changed" before going out again, i.e. adding a casual extra 45 minutes exercise to my day (I'd never do that now), and then "going out" was quite active too, we seldom sat in restaurants. Ultimately my tolerance for alcohol was much lower 9-10 years ago, so I'd have a glass and genuinely not really think to have another. It's been a long time since that happened to me now!
OK, I'll stop rambling - my point is that when I really scrutinise my lifestyle differences, they just involve being less impoverished and less sedentary, and will power just doesn't really play into it in my case. (Again, age / metabolism might be a factor, but that's not really quantifiable so I ignore it.) I simply hadn't developed the terrible habits I have now, so I didn't need to "fight" them. It's little things, like I remember packing up my room to move in with DH, it took far, far longer than I'd anticipated and by 10pm I was just exhausted and stressed, and I thought "oh I know what I need - a pizza!". It took all that time for it to occur to me and it was like a gigantic treat to galvanise me. I do it all the time now. I think some people can cruise through life without ever developing those bad habits - I think I know a few, but as you say, not very many at all.