As for calories this is quite a complicated question. As you mentioned the calories needed depend on a number of things. One of the most important things we should remember (beside of course gender, weight and age) is a physical activity level. For example, young women who are office workers getting little exercise require about 1800 calories. Young men who are vigorously active need even 2800-3000 calories.

Besides there is a question of not only the QUANTITY, but also the ''QUALITY" of calories. I mean the following. What do we eat to get the required quantity of calories?
e.g.
Person 1 eats a cream cake. This cake has approx. 300 calories/100g.
Person 2 eats a vegetable salad with olive oil (70 calories/per 100 g), steamed chicken breast (115 calories/per 100 g) and grilled vegetables (50 calories/per 100 g).

So Person 1 and Person 2 eat almost the same quantity of calories. But is the quality of these calories the same? NO.
Person 1 eats a cream cake. A cream cake consists of unhealthy FAST SUGARS and saturated fat.
Person 2 eats vegetable salad (healthy slowly-digested carbohydrates) with olive oil (healthy vegetable fat), chicken breast (protein).

So, the same quantity, but different benefits.

That is why two questions are important: how much do we eat(how much calories)? and what do we eat?