Definition: "Youngling" is a common term for Jedi initiate, a child in the first stages of Jedi training. It is also a generic, species-neutral word for a young child.
In-universe: In the early millennia of the Jedi Order, there was no strong central organization to oversee Jedi training and other policy matters. Jedi were free to choose apprentices who they felt had Force potential.
Gradually, the Jedi Order became more centralized, with power resting in the Jedi Council. After the Battle of Ruusan around 1,000 BBY, the Council established new standards for Jedi training. Seeking to avoid training anyone who risked falling to the dark side, they decided to recruit children with Force potential while they were still infants. By raising children in the Jedi temple, cutting off contact with their families and the outside world, the Jedi Council hoped to better teach them how to avoid attachment, fear, and other negative emotions that lead to the dark side.
Younglings were assigned to clans, where they learned basic Jedi principles and techniques. Once they finished their training as younglings, generally around age 13, they either became Padawans to a Jedi Knight or Jedi Master, or were assigned to the Service Corps.
Families of Force-sensitive children had mixed reactions to this new Jedi practice. Some were glad that their children could reach their full potential, but others saw the Jedi as mere kidnappers. One well-publicized controversy centered on "Baby Ludi." The Jedi initially believed her to be an orphan; when her mother appeared and tried to regain custody, they refused to give her up.
After the Jedi Purge, the Jedi Council was destroyed and Luke Skywalker had to rebuild the Jedi Order from scratch. He trained older students as a matter of necessity: there were no resources to search out Force-sensitive infants. As Luke rediscovered the practices of the old Jedi Order, he restored some of the Jedi initiate clans.
- Jedi vs. Sith: The Essential Guide to the Force by Ryder Windham (2007)
- The Jedi Path: A Manual for Students of the Force by Daniel Wallace (2010)