An interesting thing about fertility rates is that they determine not only population, but the exact young:old ratios, approximate average age, and more! Imagine you have a fertility rate of 1. That means each successive generation will be half as large as the one before it. As humans have a peak fertility window of ~20 years, this ends up being the time of a generation, making it really easy to model.

Imagine you have exactly 1 newborn in the latest generation. Then the prior generation must be composed of 2 ~20 year olds, 4 ~40 year olds, 8 ~60 year olds and 16 ~80 year olds. So you end up with a total ratio of 24:7 old (defined as 60+):young and a 24:6 or 4:1 retirement:working age (after we remove the 1 newborn). By contrast a fertility rate of 2 means each generation is the same size as the one prior so you end up with a 1:1 retirement:working ratio. And with positive population growth you have an exponential system with more workers than retirees

This is one of the main mechanisms through which fertility collapse drives economic collapse. Not only does your population shrink far more quickly than most realize, but you end up with the overwhelming majority of your remaining population being elderly. So yes - you have fewer people consuming resources, but you also have exponentially fewer people working those resources.