Although I've yet to address it in any of my stories, my "head-canon" for the world they take place in is that the human lifespan extends exponentially over a handful of generations so, my as today's 60yos resemble to the 40yos of thirty years ago, people would stay at their prime for much longer, essentially aging to about 25-30 then staying there for fifty years or so, meaning that a couple could have children, raise their to young adulthood, eat them then start again with babies two, three or even four times over, with of course the survivors of each batch maturing and having their own children.