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.