"why do people over 18 have to vote?"

Under Australian law, it is compulsory for all eligible Australian citizens who are 18 years and over to enrol and vote in federal elections, by-elections and referendums. The Australian Constitution gives the Australian Parliament the power to make laws about how elections should be run. In 1924, the Parliament passed a bill—proposed law—which made voting in federal elections compulsory. Until then voting had been voluntary, although from 1911 it was compulsory to enrol to vote.

The bill was introduced by Senator Herbert Payne (pictured) who was concerned about a big drop in the number of people voting. At the 1919 election over 71 per cent of enrolled voters cast a vote; at the 1922 election this was less than 60 per cent. Senator Payne said that a Parliament elected by only about half of all voters was making laws for all Australians. If voter participation rates continued to drop, Senator Payne was concerned the Parliament would not truly represent the will of the Australian people or make laws in the interest of all Australians. The impact of the change was immediate, with voter turnout at the 1925 election rising to over 91%.

When the bill was passed in 1924 it applied to Australians aged 21 years or old, because at the time 21 was the age at which you were considered an adult. In 1973 the Parliament passed the Commonwealth Electoral Act 1973  to lower the voting age to 18. The Australian Electoral Commission has further information about compulsory voting.

diagram to help explain why do people over 18 have to vote?