Zanu-PF and the MDC have been accused of protecting existing candidates at the expense of newcomers.
President Robert Mugabe is eager to hold elections in March, but first he has to manage a growing row between his old guard and an emerging younger crop seeking to stand for the party in the next elections.
It is a problem shared by Movement for Democratic Change leader Morgan Tsvangirai, who also faces criticism that he is trying to protect senior allies by not subjecting them to party primaries ahead of next year's general election.
Although neither Mugabe nor Tsvangirai faces any challenge to his position, they are both caught between protecting their longstanding lieutenants and allowing the new blood they need to appeal to the all-important youth vote.
Zanu-PF expects to hold primary elections in December. Its annual conference next week is expected to debate growing controversy over proposed rules that may restrict the entry of a growing group of young aspiring candidates.
Zanu-PF, whose presidential candidate, Mugabe, will be 89 in March and which is led mostly by ageing struggle veterans, has struggled to attract young voters. But it has attracted several young businesspeople eager to stand as Zanu-PF candidates for Parliament.
This is part of Zanu-PF's plan to appeal to younger voters, but it also means many of its veteran politicians may have to give way, which they are not ready to do.
"We welcome all these young and enterprising people who are coming from the corporate world to invigorate the party," Zanu-PF spokesperson Rugare Gumbo said. The "young guns" needed only to show loyalty to the party, he said.
But party administration secretary Didymus Mutasa is proposing rules that would bar any candidate who had not served in the party for at least five years. This would freeze out many of the new candidates.
Mutasa said the Zanu-PF leadership would determine whether each candidate was suitable or not.
"We have to stand as a party and choose who can stand for us. We want people who can represent us well, people who are well cultured. We won't pick just any fool," Mutasa said last week.
In some constituencies, there are up to 10 aspiring Zanu-PF candidates, party officials say.
Jonathan Moyo, a member of Zanu-PF's politburo who is considered a top strategist for the party, said the proposed rules were meant to guard the party from "the nefarious machinations of opportunists, infiltrators, merchants and agents of regime change" who would use elections to "derail, demoralise, destabilise or destroy the party".
But Moyo said the new blood, drawn mostly from security organs and business, should be allowed to stand even if they had not served Zanu-PF for long.
In the MDC, supporters accuse the leadership of trying to protect senior officials from the challenge from new aspirants.
In a recent interview with the Zimbabwe Independent, Tsvangirai said he was seeking a balance between not imposing candidates on supporters and avoiding having to "start with a Parliament full of new, inexperienced people".
Nelson Chamisa, the organising secretary of the MDC, said a sitting MP would not directly face a primary, but must go through a "confirmation process", which involved party members voting on whether to keep their MP or not.
"If they fail to garner the required support, they will go for primary elections," said Chamisa.
Brian James, hoping to stand against a veteran MDC MP in the Mutare constituency, is opposed to the proposed "confirmation process" and insists primaries must be held in all constituencies.
"People must be given the chance to choose who they want to be led by," said James.
But MDC spokesperson Douglas Mwonzora said a primary election was not the only democratic way to choose leaders, saying: "In some cases, it has produced undesired outcomes."
Political commentator Tanonoka Whande said the leadership of both parties was deliberately stifling newcomers.
"Youngsters in both parties are clamouring not only for recognition and inclusion, but [also] for accommodation and space.
"They see the shortcomings of the old-timers and how their non-performance is hurting their parties. But the leadership of both parties is reluctant and afraid to let go," said Whande.
Zanu-PF and the MDC have agreed on new regulations compelling candidates to produce a "certificate of authorisation" from their party proving they are its official candidate.
In the last election, both parties suffered split votes after candidates from the same party stood against each other, protesting the results of party primaries.