I can't seem to find any good resources on this. How old is the typical elf when they first have children? How many generations would typically be alive at one time from a single family? Is it in the range of 2-4 like with humans, or do their extended lifespans mean it's not atypical to have living great-great-grandparents?
100 to 200 years
According to Races of the Wild (2005), p.13:
Though an elf reaches mental and physical maturity at the age of 25, very few elves become parents until much later in life. Elves rarely feel that they're ready to settle down and begin families before they're at least 100 years old, and most stop having children soon after reaching the age of 200. Elf children are not as numerous as one might expect, given the length of an elf's child-rearing years, because elves are less fertile than humans and other shorter-lived races. A typical human couple might have one to four children over the course of a decade, but an elf couple might take fifty years to have the same number of children.
In a sidebar, that page goes on to say that while elves are physically mature by age 25, they aren't considered adults until around 100, and most elves do not have families until that age.
While this is a D&D 3.5 book, the lore on elven lifespan is consistent between that edition and the current D&D 5th edition.
Player's Handbook, 3.5e (2003):
An elf reaches adulthood at about 110 years of age and can live to be more than 700 years old.
Player's Handbook, 5e (2014):
Elves can live well over 700 years, giving them a broad perspective on events that might trouble the shorter-lived races more deeply.
According to the most recent official lore, elves probably bear children mostly between ages 100 and 300.
The latest official publication on 5e elves is Mordenkainen's Tome of Foes, which has an entire chapter chock-full of lore on elves, including elven life cycles. It indicates:
- Elves experience adolescence starting "in their second or third decade."
- Elves typically reach adulthood "at the end of the first century of life."
- Elves transition into "elder" years (i.e., middle-age) "in their third or fourth century."
It does not explicitly state when, in relation to those life-cycle milestones, elves tend to procreate. However, one can reason from the authors' very use of humanocentric life-cycle terms. In humans, "adolescence" is a period preceding a person's child-bearing years; "adulthood" is the period encompassing the person's child-bearing years; and middle-age is a period when child-bearing becomes increasingly difficult and unlikely, though not strictly impossible. Applying the same concepts to the long elven lifespan, their child-bearing years would begin at the end of their first century and taper off beginning in their third century.
As numerous answers state, the books are not explicit about this so much: we have physical maturity at 20-30, “adulthood” around 100, and “middle age” around 300-400. Based on human definitions of those terms, we expect child rearing to happen in the “adult” stage, and much of it towards the beginning of it. The 3.5e book Races of the Wild seems to confirm that intuition.
But it’s clear to me that this is not precisely nailed down by the books. The last time we got any kind of firm statement on the subject was two editions, and thirteen years, ago. There is room here to do things differently.
One suggestion I have seen that I like a whole lot is the idea that elves have children earlier in life—that they aren’t considered “adults” until they have raised their own children to child-rearing age, or even until they become grandparents. This jives with the 20-to-30-year-old physical maturity, followed by 70-to-80 years of ...what? before adulthood. If elves marry, or mate, or whatever, between 30 and 50, and have children and raise those children until they are themselves 30-50, before they are considered an “adult” and free to go out into the world, having done their duty by their community, that produces a markedly different culture from those of humans, gives elven adventurers a remarkably different view of life from their (mostly childless) human allies, and so forth. And I think anything that makes elves more than “snobby, tree-hugging, pointy-eared humans” is a good thing.
It also is just a fantastic question to the “what on earth have they been doing for 80 years? Why are their skills only as good as a human’s would be as a teenager?”
Until someone comes up with some interesting sources for this, we can have a look at the PHB. It says that they live up to 750 years, become physically mature around the same time as humans (let's call it 20) and claim adulthood around 100.
We cannot infer how many kids they might have, but it tells us that their generation is probably around 100 (and a bit) years long (or 20, but starting family should follow after starting adulthood, not before, I think).
Maybe about 225 years?
Based on this source, a good rough figure for a human generation is ~30 years. We'll note that's considerably greater than the minimum possible generational gap in a given family, as humans are usually reproductively capable from their mid-teens - but on average, people have kids at about 30 years old.
Humans are described as rarely living for longer than a century; elves may live up to 750 years. Proportionately, elves live about 7.5 times longer than humans. If we extrapolate and assume that elves probably decide to have children at about the same stages in their lives as humans do, that'd make an elven generation about 225 years.
However, given that elves will be mature and reproductively capable for significantly longer time periods than humans are, it would be safe to assume much greater variation in individual cases than is seen in humans. In a bloodline which consistently has children young (for an elf, at around a hundred years old and the start of their claimed "adulthood"), a living great-great-great-great-grandparent would be entirely possible. It would also not be entirely out of the question that relatives from higher up the family tree - great-uncles and great-aunts - would be of similar age to their further removed nieces and nephews, if a given elf had some children young and also had more children a couple hundred years later.