All children advance to the same age group in December at a public ceremony for each grade level.
People in The Giver do not celebrate individual birthdays. Any child born in a given year has the same birthday and the same age. Age is not an actual number, it is a category for grouping children.
Very soon he would not be an Eleven but a Twelve, and age would no longer matter. He would be an adult, like his parents, though a new one and untrained still. (ch 7, p. 51)
The fact that Jonas is number Nineteen is significant. He is one of the older children. At his naming ceremony, “he had been already standing and bright-eyed, soon to walk and talk” (ch 7, p. 50)
The numbers were rarely used after the Naming. But each child knew his number, of course. Sometimes parents used them in irritation at a child's misbehavior, indicating that mischief made one unworthy of a name. (ch 7, p. 50)
The fact that children do not get names until they are a year old and assigned to parents, and that children do not celebrate individual birthdays is all part of the Sameness concept the community embraces. Everyone is the same, and no one stands out from the group more than possible.
Names are recycled, and given by committee. When a person dies, the name is given to a baby at the next Ceremony of One.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book) (p. 51). Houghton Mifflin Harcourt. Kindle Edition.
Each December the birthdays are celebrated and there is a special ceremony for the different age groups.