Kokiri and Koroks
The Kokiri were a magical race of children that lived deep inside the Lost Woods. They never grew old but always looked like they were about ten. They lived in tree houses in a small clearing called the Kokiri Forest, where they were protected by the Great Deku Tree.
The Kokiri were small and playful. They wore green tunics and looked like ordinary children who have been playing in the dirt too long.
We don't know how exactly the Kokiri would be born, but they all called the Great Deku Tree their father. We do know however, that some of them could be twins or triplets. Every Kokiri had a guardian fairy, which was like their best friend and caretaker.
The Kokiri alone were protected from the effects of the Lost Woods, presumably by their faeries. All others who enter them, are turned into Skull Kids (in the case of children) or Stalfos (if they are adults).
The Kokiri were of the belief that by leaving the forest they would die; for this and other obvious reasons, they were the only ones who did not fight in the Hyrulean Civil War. Shortly after the war, they had a self appointed leader, a bully named Mido.
The Great Deku Tree also used to keepsake the Kokiri Emerald (one of the keys used by the ancient sages to seal the Sacred Realm) before entrusting it to Link the Hero of Time.
In Termina, we saw the Kokiri mark on Skull Kid's gloves; and there was an odd little man named Tingle, who dressed like them and thought he was a fairy. This may be proof of the species' existence somewhere in Termina too.
With all of that said, the truth behind the Kokiri's everlasting youth is that they are not what they appear to be. The true nature of the Kokiri is that of vegetables. In their real form they are known as the Koroks, and look like small tree stumps with a leaf as their face.
The Koroks typically assist the Deku Tree in preserving and nurturing the forest. They are fearful small being, and are very light which allows them to fly using the large leaves of the Deku Tree. Also one of the Koroks is given the task of learning the Wind God's Aria, and playing it during their annual ceremony so that the Deku Tree may grow new seeds for the Koroks to spread, allowing the forest to grow larger.
The Koroks took the shape of children at the end of Hyrule's Civil War to raise and offer shelter to Link, the would be Hero of Time. This may have been accomplished through the magic of faeries (that did not accompany Koroks as they did with Kokiri). In this state they had very little knowledge of the world outside the forest as well its history and religion.
Afterward, they had different fates in the two timelines. In the original one, some may have remained Kokiri while most probably went back to being Koroks and hid themselves deep in the woods.
In the new timeline, the Kokiri celebrated Ganondorfs defeat outside the forest with the other races (and it can be assumed they learned much from this contact), but they all returned to being Koroks upon the flooding of Hyrule, and isolated themselves once more. They abandoned the Lost Woods as they became far too dangerous and established themselves into forest haven which appears to be the still living body of the previous Deku Tree. From there, they travel to other islands every year to plant trees in the hope that some day their roots may create a new land.