I really do believe that it varies from person to person. I am in my early forties and I have been as little as a size 12, but I really have to work at it. So for me to remain a size 12, I eat boiled eggs and a piece of fruit or half an avocado for breakfast, chicken salad for lunch, maybe a small handful of nuts and something like stir fry or ramen for supper. No wine. Maybe 2 g and ts a week and a couple of squares of dark chocolate sometimes. Oh, and I train 4 x a week - 2 circuits/ HIIT, one powerlifting, one spin class, and walk everywhere.
If I deviate from this, I am a size 14/16 in weeks.