What can be said of the infant years of a people to whom a year is like a day, and a generation like a watch at night? One thing that can be said is that this time is not passed as the infancy of a mortal: red, screaming and tiny.
Or rather, we should say that their the infancy shall not be like that unless their adult years will be so as well.
Just as a death of long years is a stranger to them, so too the hidden people are strangers to the womb. Like a clap of lightning they are born of the need of a moment. Most are never more than a moment's being, dancing one step of the great dance of creation.
An staying-elf, however, must be nursed. They are nursed not at the edge of a breast but at the edge of consciousness (both their own and that of others). Like a dream or a figment they filter in and out through long years, generations of life. Perhaps they kiss the moors with the morning dew, or perhaps they wax with the young planted oak, or perhaps they reckon the days by the cutting of a persistent stream through hard rock.
In this time, they say the fair folk have no memories; this is a foolish lie. Even as a mortal infant cannot remember one day to the next but learns to distinguish faces, tongues, voices one from the other, bugs, hobs and nickers are have familiarity with the created things. What they lack is history. They cannot tell the story of their own origin (or indeed, follow a story told). They live an undifferentiated now to which a twisting cord loops ever back out of an impersonal never.
Those that know humans in their childhood often take descendants for ancestors or ancestors for descendants. A score or more of parents and children may wander in and out of now of the young nisse, all the same to them. Because they can be confused greatly by this, and because it is said that too much talking makes the hidden folk old before their years, some of fair adults try very much to prevent grown mortals from congress with inchoate fairies.
Mortals cannot remain ever in their infancy: not so the hidden folk. Theirs is an childhood that need never be touched by adolescence or death. Yet some of them do emerge from the liminality of their youth into an adulthood. Personalization can be slow or fast from an outside perspective, like waking from a dream on the one hand or dredging about for a memory deeply buried in the preconscious on the other.
And yet it is rare to find a mortal at the end of their short years with the easy, practiced ease of an immortal at the beginning of their long ones. They come out of their adolescence with the familiarity of two, or a dozen, or a hundred lifetimes of beholding each speck and stone and living thing in the pure immediacy that is granted to most mortals to see but a moment in each busy year if they are lucky.
The baffling patience of an elf at rest comes not from the ambiguity of their end, but the indefiniteness of their beginning.
For the immortal, "adulthood" is a trap, a net they spun out of stories and in which they have caught themselves. Loquacious or mum, you'll not find a fairy who is incautious with words. Perhaps if they find or forget the right word, they'll be able to slip the net.
Then again, perhaps not.