"Why should it (retirement) be discussed when it is not due?" he said in an interview broadcast on state television. "The leadership still exists that runs the country. In other words I am still there ... When the day comes and I retire ... I do not want to leave my party in tatters. I want to leave it intact."
Mr Mugabe claimed he is "fit as a fiddle," but appeared frail in the pre-recorded televised interview, at times stumbling over his words and slumping in his chair.
Mr Mugabe's actual birthday was yesterday when he was in Singapore for a cataract operation on his left eye, according to the president's office. Mr Mugabe returned from Singapore today and will celebrate his birthday at a sports stadium tomorrow.
Tomorrow's birthday celebrations, estimated to cost one million US dollars, will be held in a 50,000-seat stadium in Marondera, 45 miles east of Harare, where organisers said potholed streets have been repaired for the event.
Critics say Mr Mugabe will not discuss his retirement because he wants to die in office.
"The truth is we are faced with a very sick president who doesn't want to retire," analyst Ibbo Mandaza said.
Mr Mugabe's 90th birthday comes amid intense speculation on Zimbabwe's future when his grip on power loosens.
Vying to replace him are Vice President Joice Mujuru and Justice Minister Emmerson Mnangagwa.
In July, Mr Mugabe who has ruled the nation for 33 years since 1980, won disputed elections for another five-year term that will take him to age 94.
In his early years in power, Mr Mugabe expanded public education and health services that were the envy of the continent. But Zimbabwe's economy went into meltdown in 2000 after Mr Mugabe ordered seizures of thousands of white-owned commercial farms, leading to the collapse of the agriculturally based economy, once the region's breadbasket.
Unemployment has soared to an estimated 80%. Hundreds of long established industries have closed, often blaming Mr Mugabe's new black empowerment laws that compel companies to give black Zimbabweans 51% control.
Mr Mugabe has blamed the economic slump on Western economic sanctions, mostly travel and banking bans imposed on him personally and his closest associated to protest human and democratic rights violations.
In recent weeks the country has been seen allegations of massive corruption in state enterprises at a time when many Zimbabweans are surviving on less than 2 US dollars a day.