Communal rearing of the young by the prepubescent.
This,” said I, “and all that precedes has for its sequel, in my
opinion, the following law.” “What? “That these women shall all be
common to all the men, and that none shall cohabit with any
privately; and that the children shall be common, and that no parent
shall know its own offspring nor any child its parent.”
Plato's Republic, Book 5
Let us consider the elves as an extension of the system of humans. Why should humans reach reproductive fitness in their teen years? Why not as soon as they can walk, like other animals? It has been proposed that nonreproductive but competent individuals (grade school aged children) improve genetic fitness because they can watch energetic smaller children and keep them from falling in holes etc. Those individuals who, by genetic accident, delayed having their own babies turn out to improved the fitness of their genes by keeping their siblings alive. The trait spreads - evolution.
So too the elves, to the extreme. Competent prereproductive young people (ages 20-40) live in a communal kibbutz, in which all members consider themselves brothers and sisters and raised in equality (in the above quote Plato states " women shall be common to all men" but the converse is also true - he actually proposes raising boys and girls as equals). The young elves have the energy and playfulness to devote to the project of raising very young kids. Using the skills of these older immatures in this way leaves the adults available to pursue whatever grown elves do - exactly as is the case for humans.
This also would produce a more equitable and fair social structure, which is why Plato proposed it and why the Israelis tried it. Structuring the entire elf society as Plato recommends in The Republic would come out fairly fantastic, even though his plan is over 2000 years old.