While the drop in voting age will take effect in the next election, the seat bonus will only be scrapped in two elections from now.
This was because the leftist government of Prime Minister Alexis Tsipras was unable to muster yesterday the required support of two thirds of parliament, or 200 lawmakers, for scrapping the generous 50-seat bonus.
The lower voting age however did not require an enhanced majority to be passed.
A total of 179 lawmakers voted in favour of the bill, with 86 against and 16 abstentions.
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Even with the bonus in place, back-to-back elections were required in 2012 to form a government, plunging the debt-laden country into political uncertainty for months.
Tsipras won the last election in September, and the next polls are due in 2019.
Introducing proportional representation had been a long-standing pledge of his Syriza party.
However, Tsipras' coalition government rests on a fragile three-seat majority and it is widely assumed that he will be unable to complete his four-year term.
He now lags behind in the polls to Kyriakos Mitsotakis, leader of the conservative New Democracy party.