Campaigners have demanded a pre-election pledge from the three main parties to lower the voting age to 16 by the next general election.
If you can leave home and raise children, should you have a vote?
The coalition, including the National Union of Students and the Electoral Reform Society, said it would "invigorate" the youth vote.
Campaigner Joseph Ammoun, 16, said: "Young people know what they think about issues."
The Liberal Democrats have already pledged to reduce the voting age to 16.
Last year, following a 12-month review, the Electoral Commission rejected the idea of lowering the voting age.
Both the Labour and Conservative parties followed the commission's lead by ruling out a change.
The UK's 1.5m 16 and 17-year-olds would become further alienated from the democratic process if they continued to be excluded from voting, claimed the Votes at 16 coalition.
The group said that if people at 16 were able to leave home, get a full-time job, pay taxes, raise children and join the armed forces they should also be able to vote.
Lowering the voting age could also help to reinvigorate the youth vote by forcing MPs to take an active interest in the issues that concern young people, they said.
The group - which also includes the Children's Rights Alliance for England, British Youth Council and Children's Parliament in Scotland - have written to the party leaders, asking them to promise to extend the franchise.
The letter urged them to "demonstrate their faith in and respect for younger citizens by working to ensure that 16 and 17 year-olds are no longer unnecessarily denied a stake in their democracy".
Louise King, of the Children's Rights Alliance, said: "At 16 and 17, young people's lives are as rich and varied as at any other age.
"They have considerable responsibilities and routinely make complex decisions but adult society does not consider them responsible enough to vote.
"This is both illogical and unjust."
Matthew Green, Liberal Democrat spokesman for young people, said the party would allow voting and standing for office at 16.
"We believe that involving young people earlier in the political process will lead to a lifetime of interest and activism," he said.
In April 2004 the Electoral Commission recommended a further review within five to seven years to give time for more research on the social and political awareness of those at the current minimum voting age of 18.
The commission also said compulsory citizenship education in UK schools would have by then had time to develop.