In Star Wars: The Phantom Menace, Obi-Wan Kenobi (Ewan McGregor) is the Padawan learner of Jedi master Qui-Gon Jinn (Liam Neeson).
Qualified children with high Midichlorian counts are normally taken at the age of 6 months to the Jedi temple, where they recieve specialized training. At about 12 years of age they may be taken on as an apprentice (Padawan learner) by a Jedi knight or master (a Master is a knight who has already succesfully trained a Padawan).