In The Giver, 12 is not the right age to get a life long job. Why is that?

Actually, if you have a look at the text, you will see
that when the people of this community reach the age of twelve they are given their
"Assignment," when they will be clearly told what their life-long job will be. If we
look in Chapter Two, when the parents of Jonas talk to him about the Ceremony of Twelve
and what will happen afterwards, we see that this is an incredibly important moment for
individuals, as it is the last of the ceremonies, and after it, "age isn't very
important." Each child has been carefully observed from their birth and has had an
Assignment selected for them as a result of this careful and lengthy observation. After
the Ceremony of Twelve, time is only spent with other people in training and with the
"Assignment group." There are "no more volunteer hours. No more reacreation hours." Thus
it is that in the world of Jonas and the community that he inhabits, twelve is actually
chosen as the age when people are ready to begin their life-long job, having had a
special role carefully selected for them that will suit their character and gifts and
responsibilities.