Argentina's Congress has approved a law to lower the voting age to 16, in a move that will take effect for key mid-term elections next year.
President Cristina Fernandez has courted the youth vote and critics say the change is designed to boost her party's chances in the 2013 polls.
Supporters of the move say it will widen political participation.
Voting is obligatory in Argentina for people aged 18 to 70, but will be optional for those aged 16 and 17.
The long and bitter session saw deputies trade accusations and most of the opposition walk out before the measure was put to the vote.
In the end, the bill, which had already passed in the Senate, was approved by 131 votes to two.
More than one million 16- and 17-year-olds will be eligible to vote.
"It is a very important initiative because it expands the frontier of rights," said Agustin Rossi, head of the governing bloc in the Chamber of Deputies.
Supporters of the change argued that young people deserved the chance to participate in decisions affecting Argentina.
Critics have said the change is designed to increase support for President Fernandez's party in the 2013 legislative elections, with the ultimate aim of allowing her to run again for the presidency.
Ms Fernandez has a majority in both houses of Congress but would need the support of two-thirds of legislators to change the constitution to allow her to seek a third term.
She has not commented publicly on any such plans.
Across Latin America, the voting age is generally 18 but in some countries, including Brazil, Ecuador and Nicaragua, citizens can vote from age 16.