Graduation

A global tradition, graduation from school is a major landmark in the life of most, if not all, magical people. Magic schools train children in the use of magic, and teach them various other life skills, from age 12 to age 20, after which they typically go straight to work, sometimes in the form of an apprenticeship or job-specific training. Though so, will study further, pursuing higher education (either magical or Mundane), there is still a marked difference between higher education and, well, school.
Schooling in the magical world is largely unified, to ensure all schools around the globe are held to roughly the same standards. Though some variation naturally still exists, everyone everywhere is taught the same basics, from the same age. As such, all magical people, be it human or not, be it born from other magic-users or not, will join school as a child, and graduate as an adult. Because of this, graduation is often seen as not just a transition from school-life to work-life (which it isn't always), but as a transition from childhood into life as an adult.

History

Celebration of graduation as a child becoming an adult likely dates back to the introduction, or at least standardization, or magical schools. Not much is known about its exact past, but then to be fair, few have bothered to look into it. Likely, it arose separately in countless cases, a logical progression from "my child is going to school" to "my child has graduated and is now ready to work like any other adult".
Regardless of its origin, graduation is now celebrated globally by almost everyone as a transition from childhood to adult life. Similar is the celebration of people's 21st birthday, which is often their first birthday out of school, but its link is shakier, and it typically goes with a graduation celebration, rather than replacing it.