Many people think that children remain children in heaven and are like children among the angels. People who do not know what an angel is can corroborate this opinion because of the images here and there in churches, where angels are represented as children. However, things are actually very different. Intelligence and wisdom make an angel, qualities that they do not have as long as they are children. Children are with the angels, but they themselves are not angels yet. Once they are intelligent and wise they are angels for the first time. In fact—something that has surprised me—then they no longer look like children but like adults, because they no longer have a childlike nature but a more grown-up angelic nature. This goes with intelligence and wisdom.
The reason children look more grown-up as they are perfected in intelligence and wisdom—that is, like adolescents and young adults—is that intelligence and wisdom are the essential spiritual food.b So the things that nourish their minds also nourish their bodies, which is a result of correspondence, since the form of the body is nothing but an outward form of their inner natures.
It does need to be known that children in heaven do not grow up beyond the prime of youth, but remain at that age forever. To assure me of this, I have been allowed to talk with some who had been raised as children in heaven and had grown up there, with some while they were still children, and then later with the same ones when they had become youths; and I have heard from them about the course of their life from one age level to another.
from Heaven and Hell, Section 340