It's the same for my dojo. The chief instructor determines who he thinks will likely make it to instructor rank and begins easing them into a teaching role. Sometimes it's San-kyu, sometimes it's I-kyu. I'm a yon-kyu (testing for san-kyu a week from today) and I come to children's classes to help them with technique. I don't teach per se, but I'm sort of being introduced into that role.
In Nihon Goshin Aikido, a year of teaching is required for promotion to shodan. Sensei says by the time I'm preparing my shodan tests, since I've begun to learn how 3+ years out, the teaching requirement will be old hat! I've been doing it for a few weeks now, and I've noticed that helping the children with their technique has given me insight into my own. I spontaneously found the answer to a problem I always had with kote gaeshi when I was helping a struggling 9-year-old with the technique!