I read that one needs 5% fewer calories every decade which is sad news.
I have been on a diet since I was about 15 and have a very restricted intake now as, if I put on weight, I cannot lose it again without near starvation - which is horrible.
I am slim but, as I'm over 60, I haven't much else going for me. It has had ravaging effects on my face compared with chubbier folks with lovely smooth skin. I eat frugally in the week and have ordinary food at the weekend, when I also have a drinkie and some chocolate.
My DC both have their DF's chubby genes and love food and eating as does he.
I can see that from their eating and weight and my own, that calories and exercise really do have an effect. Eat fewer and stay slim, eat more and exercise less and the weight stays on.