Tolkien tar upp detta i en uppsats som heter "Laws and Customs Among the Eldar". Det finns huvudsakligen två problem:

Från "Lagar och tullar":

It might be thought that, since the Eldar do not (as Men deem) grow old in body, they may bring forth children at any time in the ages of their lives. But this is not so. For the Eldar do indeed grow older, even if slowly: the limit of their lives is the life of Arda, which though long beyond the reckoning of Men is not endless, and ages also. Moreover their body and spirit are not separated but coherent. As the weight of the years, with all their changes of desire and thought, gathers upon the spirit of the Eldar, so do the impulses and moods of their bodies change. [...]

Also the Eldar say that in the begetting [conceiving], and still more in the bearing of children, greater share and strength of their being, in mind and in body, goes forth than in the making of mortal children. For these reasons it came to pass that the Eldar brought forth few children; and also that their time of generation was in their youth or earlier life, unless strange and hard fates befell them.

History of Middle-earth X Morgoth's Ring Part 3: "The Later Quenta Silmarillion" Chapter 2: "The Second Phase" Laws and Customs Among the Eldar

Den sista punkten om vägtull på deras sinne och kropp är värt att expandera, och lyckligtvis gör Tolkien det senare i uppsatsen:

[A]ll the Eldar, being aware of it in themselves, spoke of the passing of much strength, both of mind and of body, into their children, in bearing and begetting. Therefore they hold that the fëa [soul], though unbegotten, draws nourishment directly from the fëa of the mother while she bears and nourishes the hrondo [body; later changed to the word hroä], and mediately but equally from the father, whose fëa is bound in union with the mother's and supports it.

History of Middle-earth X Morgoth's Ring Part 3: "The Later Quenta Silmarillion" Chapter 2: "The Second Phase" Laws and Customs Among the Eldar

Elver har bara ett litet antal barn på en liten tid.

Elverna förökar endast under en viss period av sina liv, som Tolkien sa (i citat):

The bad news is that elves tend to lose interest in sex after they've had kids. "With the exercise of the power (of generation), the desire soon ceases, and the mind turns to other things…they have many other urges of body and of mind which their nature urges them to fulfil." They do look back happily on the sexually-active time in their lives, though, a period of one to several hundred years.
What Tolkien Officially Said About Elf Sex; quote inside from "Laws and Customs of the Eldar", one of Tolkien's essays.

När de gör har barn finns det inte många:

Seven children was the usual upper limit in ancient times, but in future ages, partly due to varying levels of corruption in various elven societies, these norms were often exceeded rarely, even when corrupted do they ever succumb to deeds of Lust.
Elf children

Så det är en gång och gjort, medan älvorna är unga, och sedan sakta ner sakerna. För de flesta av deras liv har Elver inte unga (även relativt unga) barn.