Saying that Sihanouk was "very elderly, very weak, very demoralised," the palace said in a statement that the former king, once known for his vibrant public persona, was increasingly disturbed by "unnecessary" emails, telegrams and faxes from well-wishers.
In a separate statement, the palace pleaded with supporters "to not call on him, to not dispatch to him messages of greetings and congratulations, even on the occasion of his birthday."
Cambodia's capital Phnom Penh was decorated with only one large portrait of Sihanouk, who grandfatherly visage used to be commonplace throughout the city.
Palace officials said Wednesday he had marked his birthday with a low-key Buddhist ceremony, accompanied by family and monks.
Sihanouk, who suffers from a number of serious illnesses, including cancer, is expected to travel to China for medical check-ups during this year's Water Festival, one of the country's largest holidays which begins next month.
Sihanouk, one of Asia's longest-serving monarchs, abruptly quit the throne in October 2004 in favor of his elder son, Norodom Sihamoni, citing old age and health problems.
Despite giving up his role as king, he remains a popular figure, particularly among rural Cambodians.