It's a balancing approach. Maintain weight is multi-faceted, though it does all come down to calories in versus calories out. The problem though, that many aren't aware, is that you have this thing called individualism. Individualism comprises of gender, age and unique body characteristics. Men burn more calories than women. The older you get, the slower your metabolism becomes, thus food is digesting slower. Then you have these unique body characteristics, such as height and genetics (how your body handles specific food types). These will vary your above factors also.
The trick is that you have to find the balance. The older you get, the more important regular daily exercise becomes to help speed metabolism and burn excess calories that your body may otherwise hold onto.
That is a very basic outline though...
