How did the people of the community know how old they were?
Once they are adults, they typically do not even know how old they are. There is very little in the way of change and there aren't things like holidays to help mark the passage of time. Jonas's mom says that once you pass twelve, you do not usually know how old you are unless you care enough to go check the government records.
Before that, it's easy to know how old you are because the government has you in classes that correspond to your age. If you are 9, you are in the nines, etc.
In the book "The Giver" a child's age is measured by events in the child's life. A ceremony is held to mark the yearly transition from one age to another. Different symbols serve to demonstrate the ages of the children. On page 12 we learn that Jonas was 5 years old when his sister Lily was given to the family in a ceremony. Throughout the second chapter the different ages are mentioned. The father talks about his eleventh year. He talks about his sister getting her bike at nine years of age. However, once a child reaches twelve there does not seem to be a ceremony to mark any other age. The child moves into training for his adult responsibilities and out of school. He will soon assume his job position following his internship.
The adults do not seem to follow any days of their aging process. I am sure they could count back the years based on having the children in their lives and how many years have passed since their own ceremonies. There is evidence that there are elders, but no ages are mentioned. Once the children are raised the parental units will be sent to live with others whose children have been reared. The sense of age loses any value until it is time to be released. Release is probably determined by their uselessness than by a specific age.