Why do people in jonas's community have the same birthday?

Since the babies are not all the same age biologically, some kids have an advantage. 

Some were already walking, wobbly on their unsteady legs; others were no more than a few days old, wrapped in blankets, held by their Nurturers. (ch 2, p. 11)

Although it is claimed that the differences disappear by the time they are school age, there might be some advantage to being an older One versus a younger One.  Jonas was a little bit older, and he notes that it had given him a “slight advantage” when he was younger (ch 7, p. 50).  Since he is more sensitive and mature than the others, perhaps the advantage was not slight or temporary.

There are fifty children born each year.  They are given numbers from 1 to 50.  Their parents refer to them by number and not by name whenever they are frustrated with them, kind of like how our parents call their children by their full names.

Sometimes parents used them in irritation at a child's misbehavior, indicating that mischief made one unworthy of a name. (ch 7, p. 50)

For this reason, kids may not like hearing their numbers.  However, having a number is just part of their lives.  Since they are taught from a young age to not value individuality, it would not concern them to have a number.

The use of numbers and the abandonment of birthdays were both attempts by the community to institute Sameness.  This way, no one is singled out.  No one person is celebrated, except when the Ceremony of Twelve gives them assignments.  Everyone works for the good of the community, and the self is unimportant.

i read this book a lot so I mentioned the pg

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answer; peter zenger;