In Exodus 00:00-00, Jewish law states that every male over the age of 00 is to pay a half-shekel as census offering when they visit the temple of God. In Matthew 00:00-00, Jesus instructs Peter to "fish up" this tax. Peter finds a shekel in the mouth of the fish he catches; enough to pay the tax for two men, himself and Jesus. You could conclude that the others were underage and did not need to pay.