MrsK - when I was young, and up until around the age of 30, I was just like you. I could eat and drink anything I liked and stayed the same size/weight. But gradually, as I got older, my weight started to creep up.
In part I think it's due to the fact that your metabolism slows down. For me, the levels of exercise that I was involved in also dropped dramatically. And then as my disposable income grew, I was also going out more - eating out and drinking. And then started buying wine to drink at home (something I simply couldn't afford to do when I first started working!)
I didn't even gain weight with any of my pregnancies. I put 10lbs on with DS2 (he was born at 36 weeks, so I suppose I might have put more on if I went to full term), and 10 days after he was born I'd lost that!
But what I did do, as I realised that the weight was creeping on (and I mean creeping), was to do what we're all told we're supposed to do - I started watching calories and avoiding fatty foods. The inevitable upshot would have been that the carbs I was eating would have increased.
I remember in the 80s being told that pasta wasn't fattening, it was the sauce that you put on it that was fattening. This was an absolute revelation as, all during the 70s (when I was a teenager) I remember being told that pasta (and potatoes/bread) were things you should give up if you were dieting. Although I wasn't dieting at all then, my mum was - constantly.
I get quite cross when I think about it, that the advice I took very seriously left me either hungry, obsessing about calories (bowl of boiled celery anyone?) or eating foods that we now know are the ones that have the potential to do most harm.