@kymcclenahan
As @PipMenace said, just click on a mailbox to enroll, or you can enroll through the computer University option too. But you also have to submit an application and receive an acceptance letter first before the enroll option will appear on either the mailbox or computer. Teens have to wait to submit the application until about 5 days before they age up to YA. Prior to that, the option will not show up.
But if you have High School Years, teens get the submit application and enroll options if they graduate from high school early. I think the options will show up right after you graduate so no waiting. Once accepted and enrolled, they can attend University classes and graduate as teens. The only thing I found that teens can't do in University is to join the University organizations.
Hope this helps.