These numbers correspond to their ages. The numbers also represent what stage of development they are in according to the community.
Lily is only a seven, so at her Ceremony of Sevens she received a front buttoned jacket, indicating she was now old enough to button up her coat on her own. At her next ceremony, the Ceremony of Eights, she will receive a coat with pockets, which indicates she will be old enough to carry her own small things. It indicates burgeoning responsibility.
Jonas is almost done with being a child, as he is eleven, and adulthood in the community begins at twelve. At his Ceremony of Elevens he received some new clothes and a calculator. At the Ceremony of Twelves, though, he will receive his job assignment as the Receiver of Memories.
Note that even their birthdays are celebrated as community events—everyone has the same birthday—and each person is a number; rather than saying "I am seven," one would say, "I am a seven," indicating they are part of a larger group and not an individual.