Katie Acheson, CEO of advocacy group Youth Action, thinks that's hypocritical and is is calling on the government to look at lowering the voting age for the upcoming plebiscite on same-sex marriage.
"Australia should not be divided by those that can vote and those that can’t on key issues," Acheson told BuzzFeed News.
She argues lowering the voting age has the potential to change the outcome of the plebiscite.
"Young people have told us that marriage equality is in their top three issues, so empowering those 600,000 extra voters to have their voices heard in the upcoming plebiscite is essential in making this a productive, definitive and democratic process."
The federal Marriage Act allows 16-year-olds to marry in certain circumstances, and Acheson believes locking them out of the same-sex marriage debate is disrespectful because it's an issue that directly affects their lives.
Lowering the voting age is not unprecedented: Austria, Germany, Norway, Switzerland, the Philippines, Argentina, Nicaragua, Brazil and Ecuador all allow 16-year-olds to vote. Scotland gave 16-year-olds the right to vote in the 2014 plebiscite on independence, and the former Rudd government considered a similar move in 2009.
Youth Action research shows that the younger people are when they first vote, the more likely they are to continue voting throughout their lives.
Before the election, opposition leader Bill Shorten promised a Labor government would reduce the voting age to either 17 or 16, arguing that young people deserved the right to vote for laws that will change their lives.
Shorten estimated in 2012-13 more than 17,000 Australians under 18 paid around $41 million income tax (plus GST), or a quarter of the cost of the same-sex marriage plebiscite.
"My message to the Liberal party is: let's trust our young people because they're the people who are going to have to deal with the decisions that we're making right now," he said in a 2015 speech.
But Labor hasn't declared whether or not it will support the plebiscite when the legislation enters parliament, and it doesn't have a position on lowering the voting age.