Varies with the club and specific activity. The first string on our 4-H team are our junior leaders. Anyone 13 years of age or older may be a junior leader. There are many jobs junior leaders can do better than adult leaders because they are closer to the age of the members and can usually relate to the younger 4-H'ers on their own level. Junior leaders need to be given responsibility and then trusted to get the job done.
Junior leaders are really leaders in training.
Anyone in training can be expected to make a few mistakes. Most of us learn more from our failures than our successes. Don't be afraid to let a junior leader make a mistake as that's part of the training process too. Junior leaders often make excellent project and activity leaders. The most important thing to keep in mind when working with junior leaders is that these members are really young adults and are very capable of performing a wide variety of tasks.