The Ceremony of Twelve that occurs every December is incredibly important because, for Jonas and those other members of the community who are eleven, it represents their final ceremony and, in a sense, their move towards adulthood with all the responsibility that this brings and away from childhood. When Jonas speaks to his parents about his apprehension concerning the ceremony, they explain to him about the importance of this Ceremony:
After Twelve, age isn't important. Most of us even lose track of how old we are as time passes, though the information is in the Hall of Open Records, and we could go and look it up if we wanted to. What' simportant is the preparation for adult life, and the training you'll receive in your Assignment.
As part of the Ceremony of Twelve, every child is given their Assignment, which represents the role in the community that they will have for the rest of their lives. It therfore represents the culmination of childhood and also the start of their new future and what the rest of their lives are going to look like.
It is the coming of age ceremony in the community. Coming of age is a young person's transition from childhood to maturity and being accepted as a mature member of a society. Though twelve is an early age for a person, it is twelve here in the comunity. They are assigened their duty (or we can call their function as they will function in the community in this position after that point). After this ceremony their responsibilities grow and become more serious. After this ceremony they do not celebrate their birthdays collectively with a ceremony as age does not matter or even counted after that. I think Jonas and others coming of age do not prove that they are ready to be adult and not mature enough for their tasks as they receive training and keep on attanting the school courses as well. Jonas reaches his maturity only after receiving a great deal of memories. The ceremony is important as year by year they prepare new generations for the stability and sustanibility of the community.