Mr Mugabe has ruled Zimbabwe since 1980, and his current term in office is due to end in 2008.
In a hint that he might then stand down, Mr Mugabe said his Zanu-PF party was capable of choosing his successor.
The official birthday celebration will be a party for thousands of youths in the eastern city of Mutare on Saturday.
"The other day they said in Singapore my bones were not exactly of a boy of 26 but they said certainly of someone 30," Mr Mugabe said in an interview broadcast on state television.
"I feel like a 28-year-old."
The president said Zanu-PF was "capable of electing a successor as long as aspirants campaign properly and people rely on leaders who come through [the party] congress," the official Herald newspaper reported.
"There is time to campaign, but campaign at the right time and not become divisive and over-ambitious, with secret meetings taking place and denouncing and denigrating others and so on," Mr Mugabe said.
Several top Zanu-PF officials were suspended last year as factions formed around rivals to succeed Mr Mugabe.
The Herald described Mr Mugabe as "the greatest hero ever to grace Zimbabwe and Africa", and published a 16-page supplement of birthday messages.
"May your visionary selflessness, dedication and shrewd acumen continue to inspire us towards land utilisation for maximum productivity, food security and enhancement of the quality of life for farmers," said the Zimbabwe Farmers Union, which mainly represents small-scale black farmers.
Mr Mugabe's critics say his land reform programme has ruined Zimbabwe's economy.
Food, fuel and foreign currency are all in desperately short supply, and last month inflation topped 600%. His supporters blame the economic decline on Western sanctions.