Young people should have been allowed to have their say in the general election, the Children's Commissioner for Wales has said.
On Tuesday an amendment to lower the voting age was not voted on before MPs decided to go to the polls in December.
Sally Holland said many teenagers would be "champing at the bit" for a vote.
But the UK government had said it had no plans to lower the voting age and 18 was widely recognised as the age of adulthood.
Sixteen-year-olds in Bridgend who spoke to BBC Wales were split on if they should have the right to vote in any upcoming election.
But Prof Holland said she was disappointed votes for 16 was not debated in the Commons.
"Increasing the range of voices in an election debate can only be a good thing for our democracy, a democracy which our young people should be able to play a full part in as equal, active citizens," she said.
Across the UK in September, when there were rumours of a snap general election, nearly 200,000 people applied to register to vote in just 72 hours - with more than half being under 35.
But according to the latest analysis of the electoral roll, tens of thousands of people aged 18-34 in Wales are not yet registered to vote.
The Electoral Commission says between 410,000 and 560,000 potential voters are missing off the register across Wales, with younger people, renters and students the least likely to be registered to vote.
Jess Blair, director of Electoral Reform Society Cymru, said the figures should "sound the alarm for anyone who cares about democracy".
But she said lowering the voting age ahead of the December election would have caused problems with registrations, especially for those who are not yet 16 but will be on polling day.
"While it is brilliant that politicians are showing support for extending the vote to 16 and 17-year-olds, thought needs to take place about how this practically might work to stop young people being even more turned away from our democratic system," she said.
On Tuesday, a bid by Labour MP Stephen Doughty to lower the voting age to 16 failed in the Commons as Boris Johnson's plans for a December election were approved by MPs.
The Cardiff South and Penarth MP's amendment was one of a number not selected for debate by the deputy Speaker.
The prime minister's official spokesman had told the BBC earlier in the day that it would have been "administratively impossible" for the general election bill to be amended to lower the voting age.
He added: "Eighteen is widely recognised as the age people become an adult. Below the age of 18 you are treated as a minor in both the foster care system and the criminal justice system.
"Full citizenship rights including voting should be gained at adulthood."
'It is only your opinion that matters'
Trinity Davies, 16, from Bridgend, said while teenagers wanted a voice it "would not be heard" by adults as they were not taken seriously.
She worried a lack of understanding and engagement in politics could see people vote for random candidates, or just vote how their parents tell them.
"We lack knowledge of political arguments; if I'm being honest I myself don't even understand what is going on," said the Bridgend student.
"You shouldn't be asking your parents, it is only your opinion that matters, there are consequences to these votes.
"We are still seen as children; we cannot buy alcohol, drive a car or buy cigarettes," she said.
'We should be able to vote on our futures'
Gethin Edwards, from Port Talbot, said 16-year-olds needed to have their voices heard in elections which would affect their futures, especially over Brexit.
He pointed to climate change activist Greta Thunberg, saying "some 16 year olds are more mature than those in their twenties".
"We can get a part-time job in a shop or restaurant, you can do so many things when you turn 16," said the travel and tourism student.
"You are making big decisions about your life, but you cannot vote about your future."
Currently you have to be 18 to vote in all parts of the UK, apart from in Scotland where the voting age was lowered for the 2014 independence referendum.
In Wales changes to electoral rules are currently making their way through the Senedd to lower the voting age to 16 for the next assembly election in 2021.
And next month the Welsh Government is due to introduce a new local government bill, which has provisions to allow 16 and 17-year-olds to vote in local council and community council polls.
Under-18s can also vote in Guernsey, Jersey and the Isle of Man.