@Glomoro : Yeah, they do like that sort of thing. Kinda shows that most fantasy authors don't really understand genetics nor evolution.
Fun Fact #1 : Generational turnover has a much higher selective priority than long lifespan. Living longer and breeding less is just bottlenecking genetic perpetuation, and only really comes about if the complexity of the lifeform (which requires long gestation periods and thus longer lifespan) provides enough competitive advantage to compensate for the lower spread. Simply put... elves are highly unnatural.
Fun Fact #2 : The only physical limitation on breeding frequency in real-life organisms tends to be resource cost. i.e. How much of the body has to be used up to generate a child and bring it to adulthood. Given elves have almost identical bodies to humans, there is no biological reason why their reproductive rate would be any lower than a human's... well, unless I suppose they had some necessary difference in their diets that gave them super magic powers or whatever at the cost of being exceptionally rare... but I can't imagine that ever being advantageous enough to spread to fixation given the obvious downside.
Sorry about this. I tend to overthink these things a lot.