EA-style longtermism focuses on the idea that we can make life better for the trillions of humans who will come after us.

Given that rejuvenating cells and making them younger is possible, it seems extremely likely that we will eventually cure biological ageing (especially if we align AI), and we will have single generations where individuals will remain  youthful  for millions of years.

I think this will affect fertility rates and our estimates of how many ancestors we will have, but I don't know how we should go about predicting the effect.

It feels easy to imagine worlds where this increases and worlds where this decreases the fertility rate. 
 

It also seems plausible that the fertility rate becomes close to zero.

I don’t think this consideration is important for longtermism under total utilitarianism, but it is relevant for ethical frameworks concerned with the distribution of utility between individuals.